Use -fexternal-interpreter

[mpickering]

There are some quite big advantages to using -fexternal-interpreter related to #1171

GHC devs: https://mail.haskell.org/pipermail/ghc-devs/2019-May/017655.html

https://gitlab.haskell.org/ghc/ghc/wikis/remote-ghci

https://simonmar.github.io/posts/2016-02-12-Stack-traces-in-GHCi.html

One disadvantage is that plugins don't work but this option can be disabled/enabled with a flag.

cc @DanielG

[alanz]

I have always had in mind to use this to provide REPL functionality, as there is a GHC Hook to be able to provide our own file descriptors for stdin/stdout/stderr, which means we can separate out the hie use of these from the running code use of it. It also means we can keep the basic hie operation responsive, even if a user is running something intense in the interpreter.