basic/logarithm: add popcount() wrapper

basic/logarithm: add popcount() wrapper

__builtin_popcount() is a bit of a mouthful, so let's provide a helper.
Using _Generic has the advantage that if a type other then the ones on
the list is given, compilation will fail. This is nice, because if by any
change we pass a wider type, it is rejected immediately instead of being
truncated.

log.h is also needed. It is included transitively, but let's include it
directly.

macro.h is *not* needed.