Revert "Simplify rescue stack management; ref #3683"

This reverts commit eb5a606fe2 and
079f310fbc.
The rescue stack works differently from ensure stack, so the change
caused #3686 and #3688. It might take long to solve the problems,
so that I would revert the changes for now. Fix #3688

include/mruby.h

@@ -112,7 +112,7 @@ typedef struct {
   struct RProc *proc;
   mrb_value *stackent;
   int nregs;
-  int rpos;
+  int ridx;
   int epos;
   struct REnv *env;
   mrb_code *pc;                 /* return address */
@@ -141,7 +141,7 @@ struct mrb_context {
   mrb_callinfo *cibase, *ciend;
 
   mrb_code **rescue;                      /* exception handler stack */
-  int rsize, ridx;
+  int rsize;
   struct RProc **ensure;                  /* ensure handler stack */
   int esize, eidx;