Convert from hex at compile time

[lehins]

This PR is meant to be a replacement for the approach taken in #294

Implement IsString instances for a Q (TExp (PinnedSizedBytes n)) and Q (TExp (Hash h a))

This allows us to safely construct PinnedSizedBytes and Hash at compile time when we know the value statically (such as unit tests for example). Here are a few examples that were included in the haddock:

>>> import Cardano.Crypto.PinnedSizedBytes
>>> :set -XTemplateHaskell
>>> :set -XOverloadedStrings
>>> :set -XDataKinds
>>> print ($$("0xdeadbeef") :: PinnedSizedBytes 4)
"deadbeef"
>>> print ($$("deadbeef") :: PinnedSizedBytes 4)
"deadbeef"
>>> let bsb = $$("0xdeadbeef") :: PinnedSizedBytes 5
<interactive>:9:14: error:
    • <PinnedSizedBytes>: Expected in decoded form to be: 5 bytes, but got: 4
    • In the Template Haskell splice $$("0xdeadbeef")
      In the expression: $$("0xdeadbeef") :: PinnedSizedBytes 5
      In an equation for ‘bsb’:
          bsb = $$("0xdeadbeef") :: PinnedSizedBytes 5
>>> let bsb = $$("nogood") :: PinnedSizedBytes 5
<interactive>:11:14: error:
    • <PinnedSizedBytes>: Malformed hex: invalid character at offset: 0
    • In the Template Haskell splice $$("nogood")
      In the expression: $$("nogood") :: PinnedSizedBytes 5
      In an equation for ‘bsb’: bsb = $$("nogood") :: PinnedSizedBytes 5

and for Hashes

>>> import Cardano.Crypto.Hash.Class (Hash)
>>> import Cardano.Crypto.Hash.Short (ShortHash)
>>> :set -XTemplateHaskell
>>> :set -XOverloadedStrings
>>>  let hash = $$("0xBADC0FFEE0DDF00D") :: Hash ShortHash ()
>>> print hash
"badc0ffee0ddf00d"
>>> let hash = $$("0123456789abcdef") :: Hash ShortHash ()
>>> print hash
"0123456789abcdef"
>>> let hash = $$("deadbeef") :: Hash ShortHash ()
<interactive>:5:15: error:
    • <Hash blake2b_prefix_8>: Expected in decoded form to be: 8 bytes, but got: 4
    • In the Template Haskell splice $$("deadbeef")
      In the expression: $$("deadbeef") :: Hash ShortHash ()
      In an equation for ‘hash’:
          hash = $$("deadbeef") :: Hash ShortHash ()
>>> let hash = $$("123") :: Hash ShortHash ()
<interactive>:6:15: error:
    • <Hash blake2b_prefix_8>: Malformed hex: invalid bytestring size
    • In the Template Haskell splice $$("123")
      In the expression: $$("123") :: Hash ShortHash ()
      In an equation for ‘hash’: hash = $$("123") :: Hash ShortHash ()

Other safety fixes:

• Remove IsString (PinnedSizedBytes n) instance
• Fix psbFromByteString to fail on invalid input

[tdammers]

Looks good to me.