Support for annotations

[chengluyu]

I'm trying to support annotations. As we discussed, recursive pattern normalization/compilation should be an opt-in feature and can be enabled by an annotation. For example,

fun foo(xs) = if xs is
  @compile EmailList(values) then ...
  ...

I have only completed parsing as the first step. I am still not sure how it should be handled in the Elaborator, so I put this PR in draft status.

[LPTK]

For reference, the PR where this was implemented in the previous version of the compiler by @CAG2Mark: #210

[chengluyu]

Summary

• Add case Annotated(qualifier, target) to both Tree and Terms.
• Only annotations for declarations are stored in a new field annotations added to LetDecl, TermDefinition, and TypeLikeDef.
• Annotating expressions and missing annotation targets will raise a warning. In such cases, annotation qualifiers will be discarded and not elaborated.
• Annotation qualifiers must be a simple expression of selections or identifiers (or applications to them).

[chengluyu]

Please squash and merge after review. Thanks.

[LPTK]

Thanks, it looks mostly good, but:

• You should probably warn about useless annotations in Lowering rather than Elaborator – indeed, the lowering will later make use of some annotations, like @tailrec.
• Similarly, in Lowering, you should warn against useless annotations even on definitions (like class and value definitions).

[chengluyu]

It seems that there are still some warnings about unused annotations that need to be addressed in the elaborator, because by the time they reach the lowering phase, these trees are no longer present, such as open. Therefore, I kept the warnings in the elaborator when processing blocks.

[LPTK]

LGTM, thanks.

I'll merge after you the last conversation is resolved.