<xutility>: Teach _Equal_memcmp_is_safe about contiguous iterators and ranges::equal_to #819
Description
The _Equal_memcmp_is_safe machinery in <xutility> determines when comparisons in algorithms can be optimized in terms of memcmp rather than using a simple loop. That machinery is activated in a very limited set of circumstances, currently only for iterators of pointer type using std::equal_to to compare elements of integral type. To provide parity for the ranges algorithms, this machinery needs to understand std::ranges::equal_to. To provide a better experience for ranges, this machinery also needs to activate for contiguous_iterators that aren't pointers.