turbo-tasks: unroll TaskStorage iteration patterns per variant
Replace every `for field in lazy_iter()` / `match field { LazyField::V(_) =>
... }` pattern in the macro-generated code with an unrolled per-variant
sequence that uses the typed accessors (`get_<name>`, `set_<name>`,
`take_<name>`, `<name>_mut`, etc.). After this, codegen for:
* `encode_meta` / `encode_data`
* `clone_snapshot`
* `restore_meta_from` / `restore_data_from`
* `cleanup_after_execution`
* `drop_partial`
no longer iterates `&LazyField` references, so the underlying lazy storage
container can be swapped (next: TinyVec<LazyField> → packed byte tail with a
u32 presence bitmap) without touching any of those patterns again. The
closures-over-&LazyField in `find_lazy` / `set_lazy` / `take_lazy` / etc.
remain — they're still used by the per-variant accessor bodies and will be
addressed in the layout swap.
Drops the now-unused `LazyContents`, `take_lazy_contents`,
`lazy_extend_owned`, `lazy_retain_mut`, `lazy_cleanup`,
`build_lazy_index`, `restore_lazy_field`, and `gen_lazy_match_arms` /
`gen_drop_lazy_match_arm` helpers in their place.
All 50 backend tests pass; whole workspace builds.
