libstdc++: Move std::__niter_base and std::__niter_wrap to stl_iterator.h

Move the functions for unwrapping and rewrapping __normal_iterator
objects to the same file as the definition of __normal_iterator itself.

This will allow a later commit to make use of std::__niter_base in other
headers without having to include all of <bits/stl_algobase.h>.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (__niter_base, __niter_wrap): Move
	to ...
	* include/bits/stl_iterator.h: ... here.
	(__niter_base, __miter_base): Move all overloads to the end of
	the header.
	* testsuite/24_iterators/normal_iterator/wrapping.cc: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>