Find alternative to parsing GHCi output

Currently `haskell-mode` seems to rely on a pile of rather fragile regular expressions for getting information out of GHCi. However, GHCi is not intended to be a language server; rather, it is a REPL for use by humans. As such, the GHC developers reserve the right to change output freely to improve `ghci`'s usability for its intended purpose.

Unfortunately, when we exercise that right we end up with situations like #1553. The solution is here is clear: let's define a machine-readable interface. Perhaps it will be part of `ghci` or perhaps not. But let's begin the discussion. What does `haskell-mode` need out of GHCi to work robustly and avoid parsing-by-way-of-regular-expression?

In the case of #1553 I have introduced a flag reverting the output to the previous behavior. However, be aware that I would like to deprecate this flag soon; let's starting feeling out alternatives so I can do that.