Minor docstring cleanups
Remove obsolete `turn-on-haskell-font-lock` function

...which has been deprecated since at least 2004.
Cleanup ghc-core.el and define customization group/variables

Create new customization group `ghc-core` containing the two new
customization variables `ghc-core-program` and `ghc-core-options`.
Make `ghc-core-create-options` an obsolete alias of
`ghc-core-options`.