Reintroduce mrb_static_assert; #5051

Note that the home brew version of `mrb_static_assert` only works within
the function body.  This reverts commit 8f99689.

src/gc.c

@@ -1619,6 +1619,9 @@ mrb_init_gc(mrb_state *mrb)
 {
   struct RClass *gc;
 
+  mrb_static_assert(sizeof(RVALUE) <= sizeof(void*) * 6,
+                    "RVALUE size must be within 6 words");
+
   gc = mrb_define_module(mrb, "GC");
 
   mrb_define_class_method(mrb, gc, "start", gc_start, MRB_ARGS_NONE());

src/string.c

@@ -2926,6 +2926,9 @@ mrb_init_string(mrb_state *mrb)
 {
   struct RClass *s;
 
+  mrb_static_assert(RSTRING_EMBED_LEN_MAX < (1 << MRB_STR_EMBED_LEN_BIT),
+                    "pointer size too big for embedded string");
+
   mrb->string_class = s = mrb_define_class(mrb, "String", mrb->object_class);             /* 15.2.10 */
   MRB_SET_INSTANCE_TT(s, MRB_TT_STRING);