chore(turbo-tasks-backend): Clean up internal macros (#71061)
Figured it was easier to fix these myself than to try to suggest changes
on #70798:
- Fix some inconsistency inside `iter_many` about how derefing happens
depending on how arguments are used.
- Simplify `get_many`'s argument passthrough to `iter_many` using a `tt`
group:
https://veykril.github.io/tlborm/decl-macros/patterns/tt-bundling.html
- Manually line-wrap macros, which rustfmt does not do.
- Rename some of `iter_many` arguments (the name "`value`" was a little
too overloaded here) and add some really basic documentation.
- Eliminate a few branches using `?` repetitions instead:
https://veykril.github.io/tlborm/decl-macros/macros-methodical.html#repetitions
- The `value_pattern` (previously `value_ident`) of `iter_many` can
technically be any pattern (even though nothing currently needs that),
so broaden the type from `ident` to `tt`.
