Change _aaNew to return Impl*, not AA

[ntrel]

_aaNew was added recently in #3863.
This makes it consistent with _d_assocarrayliteralTX also returning Impl*.

I'm also hoping this will fix the test failures for some platforms with dlang/dmd#14257 as I'm using el_bin(OPcall, TYnptr not TYstruct - couldn't get the latter to work.

Also flesh out docs for _d_assocarrayliteralTX.

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