Rollup merge of #51664 - jebrosen:pub_parse_methods2, r=Mark-Simulacrum
Rollup merge of #51664 - jebrosen:pub_parse_methods2, r=Mark-Simulacrum

make more libsyntax methods public

Followup for #51502, which was sufficient only for the latest stable release of Rocket. The `master` branch uses a few more. I plan to reimplement the deleted method `parse_seq` in Rocket (see SergioBenitez/Rocket#666), rather than resurrecting it in libsyntax.

r? @Mark-Simulacrum

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