Implement `proc_macro::ToTokens` under the `nightly` Cargo feature

The standard library now has [`proc_macro::ToTokens`](https://doc.rust-lang.org/proc_macro/trait.ToTokens.html) which matches [`quote::ToTokens`](https://docs.rs/quote/latest/quote/trait.ToTokens.html). Could `proc-macro2` types add a `proc_macro::ToTokens` implementation that is gated by the `nightly` feature? This should allow quoting `proc_macro2` types with `proc_macro::quote!`.