Use `proc_macro::quote!()` when stabilized.

[parasyte]

We currently use a whole bunch of string interpolation (format!()) to generate code, and the situation would be greatly improved by switching to proc_macro::quote!() when it is stable. It's behind a feature gate and tracked in rust-lang/rust#54722

We will not be adding a dependency on the quote crate, since it pulls in all of proc_macro2 and would severely regress build times.