feat: Local/GlobalState custom keys & descriptions

also:
- solve issue of requiring forward declaration of constants & types
- improve code generation for state access in some cases
- `scratch_slots` reservation fixes/improvements  
- address usage of non-utf8 file encodings
- various bug fixes, refactoring, and minor optimisation improvements

examples/amm/out/ConstantProductAMM.approval.teal

@@ -252,7 +252,7 @@ _check_is_governor:
     int 0
     byte "governor"
     app_global_get_ex
-    assert // check value exists
+    assert // check governor exists
     ==
     // amm/contract.py:266-268
     // assert (
@@ -275,7 +275,7 @@ bootstrap:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     !
     assert // application has already been bootstrapped
     // amm/contract.py:69
@@ -328,21 +328,21 @@ bootstrap:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     callsub _do_opt_in
     // amm/contract.py:80
     // self._do_opt_in(self.asset_b)
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     callsub _do_opt_in
     // amm/contract.py:81
     // return arc4.UInt64(self.pool_token.asset_id)
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     itob
     retsub
 
@@ -370,7 +370,7 @@ _create_pool_token:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     asset_params_get AssetUnitName
     assert // asset exists
     byte "DPT-"
@@ -381,7 +381,7 @@ _create_pool_token:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     asset_params_get AssetUnitName
     assert // asset exists
     concat
@@ -540,7 +540,7 @@ mint:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     frame_dig -3
     ==
     assert // asset pool incorrect
@@ -549,7 +549,7 @@ mint:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     frame_dig -2
     ==
     assert // asset a incorrect
@@ -558,7 +558,7 @@ mint:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     frame_dig -1
     ==
     assert // asset b incorrect
@@ -595,7 +595,7 @@ mint:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     ==
     assert // asset a incorrect
     // amm/contract.py:127
@@ -625,7 +625,7 @@ mint:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     ==
     assert // asset b incorrect
     // amm/contract.py:134
@@ -669,7 +669,7 @@ mint:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     uncover 2
     callsub do_asset_transfer
     // amm/contract.py:147
@@ -689,7 +689,7 @@ _check_bootstrapped:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     assert // bootstrap method needs to be called first
     retsub
 
@@ -706,7 +706,7 @@ _current_pool_balance:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     asset_holding_get AssetBalance
     assert // account opted into asset
     retsub
@@ -724,7 +724,7 @@ _current_a_balance:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     asset_holding_get AssetBalance
     assert // account opted into asset
     retsub
@@ -742,7 +742,7 @@ _current_b_balance:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     asset_holding_get AssetBalance
     assert // account opted into asset
     retsub
@@ -897,7 +897,7 @@ burn:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     frame_dig -3
     ==
     assert // asset pool incorrect
@@ -906,7 +906,7 @@ burn:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     frame_dig -2
     ==
     assert // asset a incorrect
@@ -915,7 +915,7 @@ burn:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     frame_dig -1
     ==
     assert // asset b incorrect
@@ -945,7 +945,7 @@ burn:
     int 0
     byte "pool_token"
     app_global_get_ex
-    assert // check value exists
+    assert // check pool_token exists
     ==
     assert // asset pool incorrect
     // amm/contract.py:183
@@ -993,7 +993,7 @@ burn:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     uncover 2
     callsub do_asset_transfer
     // amm/contract.py:202-203
@@ -1003,7 +1003,7 @@ burn:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     uncover 2
     callsub do_asset_transfer
     // amm/contract.py:204
@@ -1061,7 +1061,7 @@ swap:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     frame_dig -2
     ==
     assert // asset a incorrect
@@ -1070,7 +1070,7 @@ swap:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     frame_dig -1
     ==
     assert // asset b incorrect
@@ -1094,13 +1094,13 @@ swap:
     int 0
     byte "asset_a"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_a exists
     // amm/contract.py:238
     // case self.asset_b:
     int 0
     byte "asset_b"
     app_global_get_ex
-    assert // check value exists
+    assert // check asset_b exists
     // amm/contract.py:233
     // match swap_xfer.xfer_asset:
     frame_dig -3
@@ -1137,7 +1137,7 @@ swap_switch_case_0@1:
     app_global_get_ex
     swap
     frame_bury 1
-    assert // check value exists
+    assert // check asset_a exists
     b swap_switch_case_next@4
 
 swap_switch_case_1@2:
@@ -1155,7 +1155,7 @@ swap_switch_case_1@2:
     app_global_get_ex
     swap
     frame_bury 1
-    assert // check value exists
+    assert // check asset_b exists
 
 swap_switch_case_next@4:
     // amm/contract.py:245-247