Tcache: Unify bin flush logic.

The small and large pathways share most of their logic, even if some of the
individual operations are different.  We pull out the common logic into a
force-inlined function, and then specialize twice, once for each value of
"small".

include/jemalloc/internal/jemalloc_internal_decls.h

@@ -92,4 +92,13 @@ isblank(int c) {
 #endif
 #include <fcntl.h>
 
+/*
+ * The Win32 midl compiler has #define small char; we don't use midl, but
+ * "small" is a nice identifier to have available when talking about size
+ * classes.
+ */
+#ifdef small
+#  undef small
+#endif
+
 #endif /* JEMALLOC_INTERNAL_H */