make rcstrs on the heap/static slightly smaller (#93805)
### What?
Splits the previously unified `PrehashedString` (which held a `Payload`
enum of `String | &'static str`) into two separate types:
`StaticPrehashedString { value: &'static str, hash: u64 }` for atoms
produced by `rcstr!` / `make_const_prehashed_string`, and
`DynamicPrehashedString { value: Box<str>, hash: u64 }` for atoms held
in an `Arc`. The static and dynamic paths were already distinguished by
the `STATIC_TAG` / `DYNAMIC_TAG` bits in `RcStr`, so the runtime branch
on enum discriminant was redundant.
### Why?
- **16 bytes saved per heap-allocated `RcStr` value .** Dynamic atoms
drop the `String::capacity` field (which was always equal to `len` since
the contents are immutable) and because they are stored in a
`triomphe::Arc` this drops the Arc payload to 32 bytes instead of 40
which matches a mimalloc bucket (previously we were rounded up to a 48
byte bucket) so we save 16 bytes.
- **8 bytes saved per static `RcStr` value.** The linker will optimally
align our 24 byte struct.
- Removes a layer of dispatch on the hot path (`as_str`, `==`, `Hash`) —
typed deref to the correct variant instead of matching on `Payload`.
(i.o.w. one 'descreminent' traversal instead of 2)
### How?
- `dynamic.rs`: `Payload` enum removed; two structs replace
`PrehashedString`. `deref_from` split into `deref_static` and
`deref_dynamic`. `restore_arc` returns `Arc<DynamicPrehashedString>`.
- `lib.rs`: `as_str` and `into_owned` dispatch on `tag()` (STATIC vs
DYNAMIC vs INLINE) rather than `location()`. New `heap_hash_and_str`
helper for `PartialEq` and `Hash` to share the static/dynamic branch.
`into_owned`'s `try_unwrap` arm uses `String::from(Box<str>)` which
reuses the box allocation (still O(1)).
- `turbo-rcstr-macros`: emit `::turbo_rcstr::StaticPrehashedString`
instead of `::turbo_rcstr::PrehashedString`.
- Added a comment on `DynamicPrehashedString` noting the future move to
`triomphe::ThinArc` to fold the two heap allocations (Arc header + boxed
bytes) into one. Deferred because that change would make
`RcStr::from(String)` copy the bytes, invalidating the documented cheap
`String -> RcStr -> String` round-trip — wants a separate evaluation.
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