Rename `MRB_STR_NO_UTF` to 'MRB_STR_ASCII`; close #4550

In #4550, @shuuji proposed the name name `MRB_STR_NO_MULTI_BYTE` for
more precise description. Although I agree that the name name is
correct, but the flag means the string does not contain multi byte UTF-8
characters, i.e. all characters fit in the range of ASCII.

include/mruby/string.h

@@ -87,7 +87,7 @@ MRB_API mrb_int mrb_str_strlen(mrb_state*, struct RString*);
 #define MRB_STR_FSHARED   2
 #define MRB_STR_NOFREE    4
 #define MRB_STR_POOL      8
-#define MRB_STR_NO_UTF   16
+#define MRB_STR_ASCII    16
 #define MRB_STR_EMBED    32
 #define MRB_STR_EMBED_LEN_MASK 0x7c0
 #define MRB_STR_EMBED_LEN_SHIFT 6

src/string.c

@@ -260,12 +260,12 @@ utf8_strlen(mrb_value str)
 {
   mrb_int byte_len = RSTRING_LEN(str);
 
-  if (RSTRING(str)->flags & MRB_STR_NO_UTF) {
+  if (RSTRING(str)->flags & MRB_STR_ASCII) {
     return byte_len;
   }
   else {
     mrb_int utf8_len = mrb_utf8_len(RSTRING_PTR(str), byte_len);
-    if (byte_len == utf8_len) RSTRING(str)->flags |= MRB_STR_NO_UTF;
+    if (byte_len == utf8_len) RSTRING(str)->flags |= MRB_STR_ASCII;
     return utf8_len;
   }
 }
@@ -512,8 +512,8 @@ str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2)
 
   mrb_check_frozen(mrb, s1);
   if (s1 == s2) return mrb_obj_value(s1);
-  s1->flags &= ~MRB_STR_NO_UTF;
-  s1->flags |= s2->flags&MRB_STR_NO_UTF;
+  s1->flags &= ~MRB_STR_ASCII;
+  s1->flags |= s2->flags&MRB_STR_ASCII;
   len = RSTR_LEN(s2);
   if (RSTR_SHARED_P(s1)) {
     str_decref(mrb, s1->as.heap.aux.shared);
@@ -651,7 +651,7 @@ MRB_API void
 mrb_str_modify(mrb_state *mrb, struct RString *s)
 {
   mrb_check_frozen(mrb, s);
-  s->flags &= ~MRB_STR_NO_UTF;
+  s->flags &= ~MRB_STR_ASCII;
   if (RSTR_SHARED_P(s)) {
     mrb_shared_string *shared = s->as.heap.aux.shared;