Reimplement axmap_next_free() to prevent distribution skew

Reimplement axmap_next_free() to prevent distribution skew

New algorithm starts the search at the specified bit, and only
looks forward (higher index) for a free bit, wrapping around the
end of the map if necessary.  This avoids a distribution skew in
the current algorithm where the low order 5 or 6 bits of the
replacement address tend toward zero.

The first_free cached value is not updated or used. But performance
of the new algorithm is marginally faster than the old algorithm
when measured on a 200 million entry map that is half full,
averaged across searches starting with each of the 200 million
entries.

A separate commit adds new tests to t/axmap.c to confirm the
correct behavior in a variety of situations and edge cases.