Error free std.sol

[d-xo]

Tweaks the compiler and std.sol so that executing cabal run sol-core -- -f std/std.sol no longer produces any errors.

Includes the following changes:

• tweaks the parse rules around identifiers to allow _ in names
• support for hex literals in the lexer
• tweak the syntax / ast definitions for type synonyms to allow type parameters on the LHS
• adds the current lexeme to the parser error output
• updates std.sol to satisfy the current sol-core pipeline

[d-xo]

I merged main and resolved conflicts but now I get type errors :(

Cannot match types:
a with n

 - in:function fst (p : Pair(a, b)) -> a {
   match (p) {
   | Pair(a, _) =>
      return a
   }
}