Add `Tokens` newtype wrapper, `TokenKind` iterator (#11361)
## Summary
Alternative to #11237
This PR adds a new `Tokens` struct which is a newtype wrapper around a
vector of lexer output. This allows us to add a `kinds` method which
returns an iterator over the corresponding `TokenKind`. This iterator is
implemented as a separate `TokenKindIter` struct to allow using the type
and provide additional methods like `peek` directly on the iterator.
This exposes the linter to access the stream of `TokenKind` instead of
`Tok`.
Edit: I've made the necessary downstream changes and plan to merge the
entire stack at once.
