Improve diagnostics

[Y-Nak]

Improve compiler diagnostics.
This PR adds structured diagnostics and source spans in more places, so errors can show file, line, column and labels.

Remaining things that should be addressed later are:

• some old tcmError / SC0299 errors are still there
• some fallback span guessing is still used
• location tests are not perfect, they do not check every span is preserved exactly

[mbenke]

In some cases the error message is repeated, e.g.

$ esolc tmp/unsafeFromWord.solc 
tmp/unsafeFromWord.solc
error: module typecheck failed for /home/ben/work/review/tmp/unsafeFromWord.solc (no desugaring):
Type not polymorphic enough! The annotated type is:
forall a . word -> a
but the infered type is:
word -> word
in:
forall a . function fromWord (x : word) -> a

 - in:forall a . function fromWord (x : word) -> a {
   let result ;
   assembly {
      result := x
   }
   return result;
}
  ──> /home/ben/work/review/tmp/unsafeFromWord.solc:3:30
  │
3 │       assembly { result := x } 
  │                              ^ module typecheck failed for /home/ben/work/review/tmp/unsafeFromWord.solc (no desugaring):
Type not polymorphic enough! The annotated type is:
forall a . word -> a
but the infered type is:
word -> word
in:
forall a . function fromWord (x : word) -> a

 - in:forall a . function fromWord (x : word) -> a {
   let result ;
   assembly {
      result := x
   }
   return result;
}

[rodrigogribeiro]

The previous comment on the codes used is not a blocker: I just want to understand the logic behind the choice for the codes.

Great work!

[rodrigogribeiro]

Could this be an enumeration instead of a String?