Rollup merge of #59671 - matklad:lexer, r=eddyb
Rollup merge of #59671 - matklad:lexer, r=eddyb

Make some of lexer's API private

Lexer is a `pub` type, so it feels wrong that its fields are just pub (I guess it wasn't exported initially), so let's minimize visibility.

Context: I am looking into extracting rust-lexer into a library, which can be shared by rust-analyzer and rustc. I hope that a simple interface like `fn next_token(src: &str) -> (TokenKind, usize)` would work, but to try this out I need to understand what is the current API of the lexer.

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