Flat container adaptors require the iterators of underlying containers
to be random access, and it is required that random access container
iterators must support three-way comparison ([container.reqmts]/39 - /41).
As a result, we should at least avoid testing "containers" with random
access but not three-way comparable iterators for flat container
adaptors.
This patch adds a new class template `three_way_random_access_iterator`
to `test_iterators.h` and fixes some usages of `MinSequenceContainer`
with the new iterators.
`MinSequenceContainer::size` can be narrowing on 64-bit platforms, and
MSVC complains about such implicit conversion. This PR changes the
implicit conversion to explicit `static_cast`.
`min_allocator::allocate` and `min_allocator::deallocate` have
`ptrdiff_t` as the parameter type, which seems weird, because the
underlying `std::allocator`'s member functions take `size_t`. It seems
better to use `size_t` consistently.
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
