Added method cache.

To enable method cache, define `MRB_METHOD_CACHE` or
`MRB_METHOD_CACHE_SIZE`. The cache size must be power of 2.
The default cache size is 128.

The measurement:
I measured simple benchmarks found in benchmark/ directory. With
method cache enabled, we gained 6-8% performance improvement, with
97-99% cache hit rate.

include/mruby/class.h

@@ -63,6 +63,7 @@ mrb_class(mrb_state *mrb, mrb_value v)
     }\
   }\
 } while (0)
+#define MRB_FLAG_IS_INHERITED (1 << 21)
 #define MRB_INSTANCE_TT_MASK (0xFF)
 #define MRB_SET_INSTANCE_TT(c, tt) c->flags = ((c->flags & ~MRB_INSTANCE_TT_MASK) | (char)tt)
 #define MRB_INSTANCE_TT(c) (enum mrb_vtype)(c->flags & MRB_INSTANCE_TT_MASK)