Avoid implicit integer casting in `backtrace.c` and `hash.c`.

src/backtrace.c

@@ -79,7 +79,7 @@ each_backtrace(mrb_state *mrb, ptrdiff_t ciidx, const mrb_code *pc0, each_backtr
 static void
 print_backtrace(mrb_state *mrb, struct RObject *exc, mrb_value backtrace)
 {
-  int i;
+  mrb_int i;
   mrb_int n = RARRAY_LEN(backtrace);
   mrb_value *loc, mesg;
   FILE *stream = stderr;
@@ -89,7 +89,7 @@ print_backtrace(mrb_state *mrb, struct RObject *exc, mrb_value backtrace)
     for (i=n-1,loc=&RARRAY_PTR(backtrace)[i]; i>0; i--,loc--) {
       if (mrb_string_p(*loc)) {
         fprintf(stream, "\t[%d] %.*s\n",
-                i, (int)RSTRING_LEN(*loc), RSTRING_PTR(*loc));
+                (int)i, (int)RSTRING_LEN(*loc), RSTRING_PTR(*loc));
       }
     }
     if (mrb_string_p(*loc)) {

src/hash.c

@@ -222,9 +222,8 @@ static void
 ht_compact(mrb_state *mrb, htable *t)
 {
   segment *seg;
-  mrb_int i;
+  uint16_t i, i2;
   segment *seg2 = NULL;
-  mrb_int i2;
   mrb_int size = 0;
 
   if (t == NULL) return;