libstdc++: P1970R2 Consistency for size() functions: Add ranges::ssize
This defines ranges::ssize as approved in Prague. It's unclear what is
supposed to happen for types for which range_difference_t is not a valid
type. I've assumed they are not meant to be usable with ranges::ssize,
despite being usable with ranges::size.
* include/bits/range_access.h (_SSize, ssize): Define for C++20.
* testsuite/std/ranges/access/ssize.cc: New test.