Enable proc-macro2's nightly feature, to benchmark rustc's proc_macro implementation.

[eddyb]

As pointed out in rust-lang/rust#49219 (comment), we're likely not seeing much action involving the real proc_macro bundled with the compiler, because proc-macro2, by default, shims all the APIs, and interacts with the real proc_macro through Strings (which was the only stabilized API for a while).
Any perf changes, in the implementation of the extended proc_macro APIs, may go by unnoticed.

cc @Mark-Simulacrum @nnethercote

[nnethercote]

This reminds me of #239.

[dtolnay]

Proc-macro2 now automatically wraps libproc_macro on sufficiently new compilers, so the benchmarks are already measuring proc_macro performance.