Move away from haskell-src-exts

[judah]

We currently use haskell-src-exts ("hse") in proto-lens-protoc to implement Data.ProtoLens.Compiler.Combinators. However, hse is effectively deprecated:

https://www.reddit.com/r/haskell/comments/bswd7m/haskellcafe_haskellsrcexts_no_more_releases/

The recommended alternative seems to be directly using the GHC API. Luckily, the Combinators module largely abstracts away hse's ADT. So we should be able to change it without too much code churn outside of that module.

This might also be a good time to do #57 (split out Combinators into a separate library).

This change would also help reduce build times. hse is notoriously slow to compile due deriving a large number of classes.

[judah]

Done in #336.