Partially parse .proto files

@shelf this is awesome, I'm going to make the PR for it.

Relies on the Parsec parser in hprotoc-fork (and ignores a lot of
the parsed information) but works sufficient for the Person.hs test.

Issues with the ZigZag-encoded types; not sure what destination type is
appropriate.

Issues with the fixed-precision types; could intro another dependency or
could finagle Data.Fixed.

Enums unimplemented but won't take long.

No support whatsoever for unnamed types or unnamed fields.
Could just use fresh names instead.