mruby/ops.h: add new instructions OP_GETIDX and OP_SETIDX.

Which represent `obj[int]` and `obj[int]=val` respectively where `obj`
is either `string`, `array` or `hash`, so that index access could be
faster. When `obj` is not assumed type or `R(a+1)` is not integer, the
instructions fallback to method calls.

src/string.c

@@ -1146,7 +1146,7 @@ range_arg:
   return STR_OUT_OF_RANGE;
 }
 
-static mrb_value
+mrb_value
 mrb_str_aref(mrb_state *mrb, mrb_value str, mrb_value indx, mrb_value alen)
 {
   mrb_int beg, len;