this gives about 20% performance improvements on pure parsing. obviously
it'll be less on full eval, but depending on how much parsing has to be
done (eg whether nixpkgs haskell modules are included or not) it ranges
anywhere from 4% to 10% in our tests.
this has been tested with thousands of core hours of fuzz testing to
ensure that the ASTs produced by the new parser are exactly the same as
the ones produced by the old parser. error messages will
change (sometimes a lot) and are currently not perfect, but we'd rather
leave that open for improvement than having this work rot forever.
Change-Id: Ie66ec2d045dec964632c6541e25f8f0797319ee2
112fd6c971