libstdc++: Reorder declarations of std::span members

libstdc++: Reorder declarations of std::span members

I find it easier to work with this class when the declarations match the
order in the C++2a working paper.

There's no need to use long, descriptive template parameter names like
_ContiguousIterator when the parameter is already constrained by the
std::contiguous_iterator concept. This is also consistent with the
naming conventions in the working paper.

	* include/std/span (span): Reorder members and rename template
	parameters to match declarations in the C++2a working paper.