Optimize for rustc 1.20+ which can match ident faster

[dtolnay]

This performs about 10% better on my machine: the included benchmark is down from 160 microseconds to 144 microseconds.

Mentioning @nnethercote regarding dtolnay/proc-macro2#94 (comment) -- would you like to try this commit in the rustc-perf benchmark before merging?

[mystor]

If we're up for optimizing for more recent versions this way, I wonder if we should implement quote! as a proc macro on sufficiently recent versions for perf.

[dtolnay]

I would welcome a PR that implements quote as a proc macro to benchmark against.

[dtolnay]

Abandoning because I would prefer not to land this more complicated implementation in the absence of rustc-perf numbers measuring an improvement.

[nnethercote]

@dtolnay: I apologize for not responding to this back in January. I just tried the patch but it has bitrotted. Would you be able to create a new version? I will be able to benchmark it against rustc-perf if you do. Thanks.

[dtolnay]

@nnethercote sure thing, here's the PR: #135