Move `mrb_mod_s_nesting()` to `mruby-metaprog` gem from the core

mrbgems/mruby-metaprog/src/metaprog.c

@@ -657,8 +657,30 @@ mrb_mod_s_constants(mrb_state *mrb, mrb_value mod)
   return mrb_nil_value();       /* not reached */
 }
 
-/* implementation of Module.nesting */
-mrb_value mrb_mod_s_nesting(mrb_state*, mrb_value);
+mrb_value
+mrb_mod_s_nesting(mrb_state *mrb, mrb_value mod)
+{
+  struct RProc *proc;
+  mrb_value ary;
+  struct RClass *c = NULL;
+
+  mrb_get_args(mrb, "");
+  ary = mrb_ary_new(mrb);
+  proc = mrb->c->ci[-1].proc;   /* callee proc */
+  mrb_assert(!MRB_PROC_CFUNC_P(proc));
+  while (proc) {
+    if (MRB_PROC_SCOPE_P(proc)) {
+      struct RClass *c2 = MRB_PROC_TARGET_CLASS(proc);
+
+      if (c2 != c) {
+        c = c2;
+        mrb_ary_push(mrb, ary, mrb_obj_value(c));
+      }
+    }
+    proc = proc->upper;
+  }
+  return ary;
+}
 
 void
 mrb_mruby_metaprog_gem_init(mrb_state* mrb)

src/vm.c

@@ -830,31 +830,6 @@ mrb_yield_cont(mrb_state *mrb, mrb_value b, mrb_value self, mrb_int argc, const
   return mrb_exec_irep(mrb, self, p);
 }
 
-mrb_value
-mrb_mod_s_nesting(mrb_state *mrb, mrb_value mod)
-{
-  struct RProc *proc;
-  mrb_value ary;
-  struct RClass *c = NULL;
-
-  mrb_get_args(mrb, "");
-  ary = mrb_ary_new(mrb);
-  proc = mrb->c->ci[-1].proc;   /* callee proc */
-  mrb_assert(!MRB_PROC_CFUNC_P(proc));
-  while (proc) {
-    if (MRB_PROC_SCOPE_P(proc)) {
-      struct RClass *c2 = MRB_PROC_TARGET_CLASS(proc);
-
-      if (c2 != c) {
-        c = c2;
-        mrb_ary_push(mrb, ary, mrb_obj_value(c));
-      }
-    }
-    proc = proc->upper;
-  }
-  return ary;
-}
-
 static struct RBreak*
 break_new(mrb_state *mrb, struct RProc *p, mrb_value val)
 {