glib/utf8: Use SIMD for g_str_is_ascii()

This moves g_str_is_ascii() from gstrfuncs.c to gutf8.c so that we can
reuse the same SIMD code for ASCII validation.

On Apple Silicon:

 Before:  3297 MB/s
  After: 26146 MB/s

glib/gstrfuncs.c

@@ -1604,29 +1604,6 @@ g_ascii_strup (const gchar *str,
   return result;
 }
 
-/**
- * g_str_is_ascii:
- * @str: a string
- *
- * Determines if a string is pure ASCII. A string is pure ASCII if it
- * contains no bytes with the high bit set.
- *
- * Returns: true if @str is ASCII
- *
- * Since: 2.40
- */
-gboolean
-g_str_is_ascii (const gchar *str)
-{
-  gsize i;
-
-  for (i = 0; str[i]; i++)
-    if (str[i] & 0x80)
-      return FALSE;
-
-  return TRUE;
-}
-
 /**
  * g_strdown:
  * @string: the string to convert

glib/gutf8.c

@@ -1886,6 +1886,25 @@ g_utf8_validate_len (const char   *str,
   return max_len == 0;
 }
 
+/**
+ * g_str_is_ascii:
+ * @str: a string
+ *
+ * Determines if a string is pure ASCII. A string is pure ASCII if it
+ * contains no bytes with the high bit set.
+ *
+ * Returns: true if @str is ASCII
+ *
+ * Since: 2.40
+ */
+gboolean
+g_str_is_ascii (const gchar *str)
+{
+  utf8_verify_ascii (&str, NULL);
+
+  return *str == 0;
+}
+
 /**
  * g_unichar_validate:
  * @ch: a Unicode character