ENH: Micro-optimize where=True path for mean, var, any, and all

This removes a 20%-30% overhead, and thus the largest chunk of
slowdown incurred by adding the `where` argument. Most other places
have fast-paths for `where=True`, this one also should have it.

The additional argument does slow down the function versions a bit
more than this, but that is to be expected probably (it has to
build a new argument dict, at some point we might want to move this
to C, but that seems worth much more with FASTCALL logic).

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