Refactor runs_avail.

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).

include/jemalloc/internal/private_symbols.txt

@@ -394,6 +394,8 @@ pages_purge
 pages_trim
 pages_unmap
 pind2sz
+pind2sz_compute
+pind2sz_lookup
 pow2_ceil_u32
 pow2_ceil_u64
 pow2_ceil_zu
@@ -468,7 +470,6 @@ rtree_val_read
 rtree_val_write
 run_quantize_ceil
 run_quantize_floor
-run_quantize_max
 s2u
 s2u_compute
 s2u_lookup