Add haskell-src as an official machine-readable component of the Haskell standard

[Lennart Augustsson]

Please explain.  Fixity information cannot be provided unless you find
all the imported modules and process those, so I'm not sure how
haskell-src-exts could do any better than it currently does.

>
> (Examples of controversies possible in haskell-src: we have the Hs
> prefix on constructors everywhere, we can't provide fixity information
> (and the haskell-src-exts implementation of this is unsatisfactory in
> several important ways), a lot of type class instances are absent
> (even Ord!), the distribution of SrcLocs is a little awkward when
> manipulating source abstractly, and some constructors allow impossible
> values, e.g. HsLambda can contain zero patterns)