Hide size class computation behind a layer of indirection.

This class removes almost all the dependencies on size_classes.h, accessing the
data there only via the new module sc.h, which does not depend on any
configuration options.

In a subsequent commit, we'll remove the configure-time size class computations,
doing them at boot time, instead.

test/unit/arena_reset.c

@@ -77,7 +77,7 @@ vsalloc(tsdn_t *tsdn, const void *ptr) {
 		return 0;
 	}
 
-	if (szind == NSIZES) {
+	if (szind == SC_NSIZES) {
 		return 0;
 	}