Rename "fast" and "slow" popcount functions.

Rename "fast" and "slow" popcount functions.

Since we now have several implementations of the popcount
functions, let's give them more descriptive names.  This commit
replaces "slow" with "portable" and "fast" with "sse42".  While the
POPCNT instruction is technically not part of SSE4.2, this naming
scheme is close enough in practice and is arguably easier to
understand than using "popcnt" instead.

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/aWf_InS1VrbeXAfP%40nathan