Contribute to qquote macro in libproc_macro_plugin

[dtolnay]

The plan is for Macros 2.0 to rely quite heavily on quasi-quoting and there is already some basic support in the compiler/macro libraries. It is a bit in flight because rust-lang/rust#37321 moves quite a lot around, but the actual qquote macro is in libproc_macro_plugin and will stay there.

See whether it makes sense to contribute anything from this crate to that system.

cc @nrc