This PR adds a parallel version of `std::mismatch` as one of the backend
operations.
It also provides an implementation based on `__pstl::__parallel_find()`
which does the heavy lifting.
`libdispatch` and `std_thread` backends expose this implementation under
their backend tags, while the `serial` backend redirects the calls to
the serial `std::mismatch`.
4 flavours of the function are exposed: 3-legged, 3-legged with
predicate, 4-legged, 4-legged with predicate.
3-legged flavours are implemented in the `default` (`composition`)
backend by redirecting the call to the 4-legged flavours.
Included tests check that:
- Semantics of the iterator-only functions is correct.
- Semantics of the predicated functions is correct.
- Custom types are supported by the implementation.
- The functions correctly SFINAE out when the first argument is not an
execution policy.
- The `nodiscard` policy is followed.
- The `noexcept` policy is followed.
- `static_assert` verifies iterators' categories.
Part of #99938.
150f33ce28