Replace syntex with libsyntax or rustc-ap-syntax

[topecongiro]

AFAIK syntex was deprecated more than a year ago, so racer may not recognize the latest syntax. Is it possible to replace syntex with libsyntax or rustc-ap-syntax?

[kngwyu]

I'm working on my branch, though it's WIP and I have no plan to send PR to upstream.

[kngwyu]

Nightly vesrion is now ready to use, though still there's a lot of TODO.

[kngwyu]

Closed via #854