Support parsing Backpack signatures.

[ezyang]

signature is an upcoming syntax extension in GHC 8.2, which just makes it possible to replace the top-level "module" keyword with "signature". For now, I drop information about which header is used, because I didn't want to change the AST.

Signed-off-by: Edward Z. Yang ezyang@cs.stanford.edu

[mpickering]

Three things

1. There needs to be a test
2. I can't find where this is described in the user guide, please can you link me to the relevant part.
3. The shift/reduce conflicts need to be explained. It seems that there shouldn't be any with this patch.

[ezyang]

Re 2, see http://ghc.readthedocs.io/en/latest/separate_compilation.html#module-signatures

Actually, thinking about this more carefully, the shift/reduce error results in a legitimate problem parsing certain modules, which I filed at http://ghc.haskell.org/trac/ghc/ticket/13765 So I guess we need a separate parsing mode for hsig files?