[WIP - DO NOT MERGE] Add native preprocessor

[jblazquez]

@ChrisDodd @mbudiu-vmw

This is the prototype implementation for the native preprocessor proposal I posted here: p4lang/p4-spec#30

All existing unit tests pass except those that depend on the exact output of error messages, which show minor visual differences between the C preprocessor and this one. For example, the C preprocessor doesn't preserve whitespace between tokens (this one does) and it also removes comments (this one doesn't).

This PR doesn't include changes to the test data (#include -> #import, among other things) because that would clutter it too much.

The preprocessor ended up being decently small. Hopefully it gives a good idea of the effort required to implement the proposal.

[mihaibudiu]

You should probably no use Barefoot here.

[jblazquez]

Yeah, I copied this from another file and didn't notice it.

[sethfowler]

This seems like a great patch; should we move forward on this?

[ChrisDodd]

Problem is that as written, this preprocessor is quite different from the C preprocessor.

[sethfowler]

Hmm, I see. I just took a look at p4lang/p4-spec#30 where this is discussed and it seems like it sort of petered out.

If we did want to go in this direction, perhaps the easiest thing to do would be to use an existing BSD-licensed CPP implementation, which we could gradually evolve in whatever direction the P4 language design committee desired.

Obviously not a high priority thing.

[jnfoster]

Should we close this pull request since (although it has many nice features) the Language Design Working Group has not chosen to take this for P4-16?

[ChrisDodd]

This is now thoroughly out of date, so I'm going to close it.