Rollup merge of #72234 - dtolnay:default, r=petrochenkov
Rollup merge of #72234 - dtolnay:default, r=petrochenkov

Implement Default for proc_macro::TokenStream

Hopefully this is uncontroversial. The only reason we've made it this far without is that proc-macro2 snuck this in for their TokenStream.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust