Add module hierarchy Agda2Hs.Language.Haskell

[HeinrichApfelmus]

This pull request adds a module hierarchy at Agda2Hs.Language.Haskell that represents the Haskell language syntax from the viewpoint of Agda2Hs.

This module hierarchy essentially re-exports a subset haskell-src-exts, but also assimilates the old Agda2Hs.HsUtils module.

Comments

• This pull request is supposed to be a pure refactoring.
• The main purpose of this pull request is to move the codebase in a direction where we can add Haddock comments to compiled Haskell declarations, Preserve comments #114 . We follow a divide-and-conquer strategy: Before solving the problem of representing Haddock comments for Haskell declarations, we first isolate a module hierarchy Agda2Hs.Language.Haskell in which the problem and its solution are supposed to reside.
• If Consider using ghc-source-gen #274 is implemented, then it would go through Agda2Hs.Language.Haskell as well. That said, Agda2Hs.Language.Haskell re-exports the opinion of haskell-src-exts on how Haskell code looks like — if didn't re-export the opinion, then it would present an alternate opinion, essentially duplicating haskell-src-exts. The idea is that Agda2Hs.Language.Haskell is a re-export of haskell-src-exts, but with a couple of utilities that are specific to Agda2Hs added.

[jespercockx]

This looks fine to me. I'm not too excited about all the extra noise of the Hs. prefixes. However it does make things more consistent, so perhaps it's fine.

[omelkonian]

I'm happy with the file organization and more compact imports, but please revert most changes in the terms where you introduced Hs..

[omelkonian]

LGTM