Redesign the cache bin metadata for fast path.

Implement the pointer-based metadata for tcache bins --
- 3 pointers are maintained to represent each bin;
- 2 of the pointers are compressed on 64-bit;
- is_full / is_empty done through pointer comparison;

Comparing to the previous counter based design --
- fast-path speed up ~15% in benchmarks
- direct pointer comparison and de-reference
- no need to access tcache_bin_info in common case

Makefile.in

@@ -178,6 +178,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/bit_util.c \
 	$(srcroot)test/unit/binshard.c \
 	$(srcroot)test/unit/buf_writer.c \
+	$(srcroot)test/unit/cache_bin.c \
 	$(srcroot)test/unit/ckh.c \
 	$(srcroot)test/unit/decay.c \
 	$(srcroot)test/unit/div.c \