rational.c: overhaul rational operators.

- define `MRB_TT_RATIONAL`
- change object structure (`struct RRational`)
- add memory management for `MRB_TT_RATIONAL`
- avoid operator overloading as much as possible
- implement division overloading in C
- as a result, performance improved a lot

include/mruby/value.h

@@ -129,6 +129,8 @@ enum mrb_vtype {
   MRB_TT_FIBER,
   MRB_TT_ISTRUCT,
   MRB_TT_BREAK,
+  MRB_TT_COMPLEX,
+  MRB_TT_RATIONAL,
   MRB_TT_MAXDEFINE
 };