avoid copying validator data in accessor

```
    5291.227,        0.000,     5291.227,     5291.227,            1, Initialize DB
       0.436,        0.928,        0.138,       51.438,         3155, Load block from database
   11962.826,        0.000,    11962.826,    11962.826,            1, Load state from database
       6.477,        1.675,        0.037,       34.174,         3101, Advance slot, non-epoch
      76.633,        3.705,       71.106,       98.085,          100, Advance slot, epoch
      18.301,       18.593,       13.208,      149.153,         3155, Apply block, no slot processing
       0.000,        0.000,        0.000,        0.000,            0, Database load
       0.000,        0.000,        0.000,        0.000,            0, Database store
```

beacon_chain/spec/datatypes/base.nim

@@ -623,13 +623,13 @@ proc readValue*(reader: var JsonReader, value: var ForkDigest)
 static: doAssert high(int) >= high(int32)
 
 # `ValidatorIndex` seq handling.
-func `[]`*[T](a: var seq[T], b: ValidatorIndex): var T =
+template `[]`*[T](a: var seq[T], b: ValidatorIndex): var T =
   a[b.int]
 
-func `[]=`*[T](a: var seq[T], b: ValidatorIndex, c: T) =
+template `[]=`*[T](a: var seq[T], b: ValidatorIndex, c: T) =
   a[b.int] = c
 
-func `[]`*[T](a: seq[T], b: ValidatorIndex): auto =
+template `[]`*[T](a: seq[T], b: ValidatorIndex): auto =
   a[b.int]
 
 # `ValidatorIndex` Nim integration