This PR renames `String.ValidPos` to `String.Pos`, `String.endValidPos`
to `String.endPos` and `String.startValidPos` to `String.startPos`.
Accordingly, the deprecations of `String.Pos` to `String.Pos.Raw` and
`String.endPos` to `String.rawEndPos` are removed early, after an
abbreviated deprecation cycle of two releases.
This PR removes all uses of `String.Iterator` from core, preferring
`String.ValidPos` instead.
In an upcoming PR, `String.Iterator` will be renamed to
`String.Legacy.Iterator`.
This PR upstreams the Verso parser and adds preliminary support for
Verso in docstrings. This will allow the compiler to check examples and
cross-references in documentation.
After a `stage0` update, a follow-up PR will add the appropriate
attributes that allow the feature to be used. The parser tests from
Verso also remain to be upstreamed, and user-facing documentation will
be added once the feature has been used on more internals.
