libstdc++: P1976R2 Fixed-size span construction from dynamic range

This includes fixes for first, last, as_bytes and as_writable_bytes
which were missing from the paper.

	* include/std/span (__cpp_lib_span): Update value.
	(span(It, size_type), span(It, End)): Make conditionally explicit. Add
	assertion.
	(span(R&&), span(const span<OType, OExtent>&)): Likewise and relax
	constraints.
	(span::first<Count>(), span::last<Count>()): Use explicit type in
	return statement.
	(as_bytes, as_writable_bytes): Likewise.
	* include/std/version (__cpp_lib_span): Update value.
	* testsuite/23_containers/span/1.cc: Check new value.
	* testsuite/23_containers/span/2.cc: Check new value.
	* testsuite/23_containers/span/explicit.cc: New test.