libproc_macro calls are extremely expensive

[dtolnay]

Context: rust-lang/rust#65080

We should look into buffering proc_macro operations in memory to defer and batch the calls we make to proc_macro. In particular, buffering the tokens from proc_macro2::TokenStream::extend and lazily extending the underlying proc_macro::TokenStream only when necessary could be a big improvement for compile times downstream, since the code produced by quote! is dominated by those calls.