perf: inline YAML CST lists

The CST keeps the lexemes of each item part and token end in a Vec,
and most hold three or fewer. They now live inline in a SmallVec, so
the parser mostly stops allocating while it keeps yaml's in-place
list edits.

The vite lockfile formats in 12.2 ms instead of 12.7 ms. The YAML
bench corpus needs 76K allocations instead of 691K at the start of
this work.