[ elab ] Support easy collection of information during TTImp traverse

[buzden]

Description

Current applicative traversals introduced at #3130 can be used for analysis of an expression, e.g. for collecting some data. But since currently the only way to analyse expression is to analyse the result of pure, used Applicative must contain at least on level of expressions, disallowing simple stuff like Const functor for applicative traversals for collecting the data.

But once we pass the original expression along with the current data to the user function, we gain the ability to traverse the data with simple applicatives, like Const. You can see an example of such collecting traversal in the newly added test.

This is a backward-compatible change in terms of main existing functions mapATTImp and mapMTTImp. However, intermediate functions for mapM<WhateveElse> changed their signatures to support using mapATTImp' internally.

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).