Rework annotations

[SquidDev]

• Split annotations in to three kinds:

  • Ann p: Our existing annotation. Stores positions, then types of positions.
  • RawAnn p: Used for top-level statements. Always stores positions.
  • TypeAnn p: Used for types. Stores positions, and then disappears during desugaring.

• Names within types (tyvars, type constructors, and promoted constructors) now track their positions.

• Split up "annotation extraction" and "position extraction" into two separate passes. This removes the need to use generics to extract an expression's type when lowering.

• Correct the span of some nodes, to more accurately reflect the full contents.

[SquidDev]

I'm aware this is a horror-story of a commit, and does make the TC a bit uglier. Wasn't really sure of an appropriate solution though.