Replace syntex with rustc-ap-syntax

[kngwyu]

Now syntex_syntax is no longer maintained, so we have to move towards libsyntax.

Here I list up change from our current syntex_syntax 52.0 and rustc-ap-syntax 133.0 (though not all).

• visit::Visitor needs life time(now its declaration is pub trait Visitor<'ast>: Sized)
• data structure for use is completely different
• ExprKind::TupField is removed by Hygiene 2.0: Avoid comparing fields by name rust-lang/rust#49718
• we have to call scoped-tls explicitly to get names of idents
• and some changes..

By this change, we become unable to compile racer with beta or stable compiler.
So we have to decide some things.

• Should we change the package name like rustfmt-nightly?
• Should we remain current master as syntex branch?

I constructed this PR from kngwyu#1 kngwyu#2 kngwyu@40466f6 and some commits.

[birkenfeld]

Why is nightly required?

[kngwyu]

@birkenfeld
Because rustc's libsyntax uses many feature gates

[davemilter]

@kngwyu

Why rustc-ap-syntax and not https:/dtolnay/syn ?

[kngwyu]

@davemilter
For some reasons.

• We currently use syntex-syntax i.e. older version of libsyntax. So to migrate over to rusc-ap-syntax i.e. latest version of libsyntax, is not so hard.
• syn is designed for proc_macro

But, if you think there's any merit to use syn, please let me know.

[nrc]

syn does have some advantages - it works on stable and is itself more stable (changes less, so less bustage for clients). However, I think moving to syn will be a really big change and using rustc_ap_syntax is a great step for right now (in particular, rustfmt uses it, so at the least we have a couple of important tools relying on the same dependency).

[nrc]

Thanks, this is great!

[kngwyu]

I created syntex branch for just in case.
And, we have to update README before next release.