turbo-tasks: address PR #93835 review comments
- Introduce `Tag(NonZeroU8)` newtype in `lazy_tail.rs` to carry the
"1..=LAZY_N" invariant at the type level and centralize the
bit/mask helpers (`bit`, `mask_below`, `mask_above`, `table_index`).
Macro emits `LAZY_TAG_<NAME>: Tag` constants and dispatch fns take
`Tag`. `Option<Tag>` is 1 byte via the niche.
- Add a Lemire reference to `sum_padded_sizes`'s loop and note the
stdlib has no built-in set-bit iterator.
- Add unit tests for `sum_padded_sizes`, `offset_of`, the mask-
partition invariant, and the `Option<Tag>` niche.
- Rename `LazyTail::install_unchecked` -> `insert_unchecked` and
`TaskStorage::lazy_install` -> `lazy_insert`. Cross-reference
`replace_in_place` from `insert_unchecked`'s safety doc.
- Add a `const _` assert pinning `size_of::<Option<TaskStorage>>()
== size_of::<TaskStorage>()`.
- Inline imports for `TaskStorage` in `backend::mod` and
`backend::operation` (drop the fully-qualified spellings).
- Fix a stale doc reference (`TaskStorageInner::evictability` ->
`TaskStorage::evictability`) and drop a stale `CachedDataItem`
mention from the schema module docs.
- Rename `TurboMalloc::good_size` -> `get_bucket_size`.
All `backend::lazy_tail::tests` (8) and `backend::storage_schema::tests`
(16) pass under cargo test and Miri's default Stacked Borrows mode.
