Recognize ident tokens instead of doing is_ident at runtime

[dtolnay]

This is a rebase of #91.

	before	after	Δ
cargo bench	125μs	119μs	-5%
cargo build --benches && target/debug/bench-* --bench	2100μs	1860μs	-11%

[nnethercote]

I tried to benchmark this but couldn't get it to have any effect. I have a local copy of quote with this PR applied and a local copy of proc-macro2. And I changed the Cargo.toml of a couple of benchmarks (ones affected in the past by this stuff) like so:

+[patch.crates-io]
+quote = { path = '/home/njn/moz/quote' }
+proc-macro2 = { path = '/home/njn/moz/proc-macro2' }

But it made only tiny differences. I don't know if I did something wrong or if it just doesn't help.

The patch is so simple that you might as well just land it for the benchmark improvement anyway?

[dtolnay]

Thanks! Yeah this came out a lot simpler than the original patch in #91 now that we no longer need to support rustc <1.20. I agree, might as well land it.