Fix arena_run_first_best_fit() to search all potentially non-empty
runs_avail heaps, rather than ignoring the heap that contains runs
larger than large_maxclass, but less than chunksize.
This fixes a regression caused by
f193fd80cf (Refactor runs_avail.).
This resolves#493.
Use pszind_t size classes rather than szind_t size classes, and always
reserve space for NPSIZES elements. This removes unused heaps that are
not multiples of the page size, and adds (currently) unused heaps for
all huge size classes, with the immediate benefit that the size of
arena_t allocations is constant (no longer dependent on chunk size).
