Currently `std::hash<Emplaceable>::operator()` relies implicit
conversion from `int` to `size_t`, which makes MSVC compelling. This PR
switches to use `static_cast`.
In `flat.map/flat.map.access/at_transparent.pass.cpp`, there's one
value-discarding use of `at` that wasn't marked `TEST_IGNORE_NODISCARD`.
This PR adds the missing `TEST_IGNORE_NODISCARD`.
Not all the code path has been exercised by the tests for
`flat_map::emplace_hint`
Adding more test coverage.
At the same time, adding more test cases for `flat_map::emplace`
Around half of the tests are based on the tests Arthur O'Dwyer's
original implementation of std::flat_map, with modifications and
removals.
partially implement #105190
