Various cleanups: Use _STDEXT_BEGIN and _STDEXT_END

[StephanTLavavej]

Select "Hide whitespace" to see how simple this change is.

We usually use our namespace macros when defining stdext machinery:

STL/stl/inc/iterator

Lines 1468 to 1470 in f392449

	_STDEXT_BEGIN
	template <class _Ptr>
	class _DEPRECATE_STDEXT_ARR_ITERS checked_array_iterator { // wrap a pointer with checking

This PR fixes the one inconsistency in our headers. (Right now, there's no difference between the macros and directly mentioning the namespace, but in the future the macros might be trickier.)

[StephanTLavavej]

I am a greedy kitty who's speculatively mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.