Add functionality to debug in GHCI

[felixlinker]

This PR adds a monadic infrastructure to inspect and debug constraint system solving. I tried documenting it, but this is always hard for the author.

I would be glad if people could point out further options for documentation. Especially, it might be valuable to point to this means of debugging somewhere. Anybody got ideas for a good place?

[felixlinker]

PS: Commit d4149c9 was necessary to make the stack ghci parser happy. I wonder why the errors that I saw didn't appear on stack build, however, without that commit, the parser complained about No instance for GHC.Records.HasField in cases were application of . wasn't properly spaced. I suspect this comes from some field syntax that parses a.b as field access rather than function concatenation.

I suggest (as in #650) to squash and merge to avoid the funny, list commit that I will revert for other branches.