these changes now update to 0.15.1, including

• loss of usingnamespace
• loss of async/await keywords
• asmclobbers now being a struct.

The grammar is known to successfully parse almost all of the zig source (except for a few files which are exceptionally long).

Hey, sorry I missed this PR. I actually pushed updates to the grammar a couple days ago and I think this work is redundant from a quick glance. Is there anything you see that is still missing/out of sync with the implementation?

There are a couple of instances where std.zig.Parse does not implement the grammar properly (for example it does not validate multibyte UTF-8 encoding currently) but I think all deviations are issues with std.zig.Parse rather than the grammar now...