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).

test/unit/run_quantize.c

@@ -111,7 +111,7 @@ TEST_BEGIN(test_monotonic)
 
 	floor_prev = 0;
 	ceil_prev = 0;
-	for (i = 1; i < run_quantize_max >> LG_PAGE; i++) {
+	for (i = 1; i <= large_maxclass >> LG_PAGE; i++) {
 		size_t run_size, floor, ceil;
 
 		run_size = i << LG_PAGE;