Add a fastpath for rb_str_normalize_ospath

This extra check is a hotspot for path operations on
macOS.

It was added in 9962aad7b0
because of a limitation of HFS+.

But all the invalid characters are outside of ASCII range,
and most paths are ASCII, so we can optimistically check
the coderange instead.

Most `rb_str_normalize_ospath` were first checking for ASCII
range, but a few like `rb_dir_getwd_ospath` in `dir.c` or
`ospath_new` in `file.c` didn't.

file.c

@@ -380,9 +380,15 @@ rb_str_normalize_ospath(const char *ptr, long len)
     const char *p = ptr;
     const char *e = ptr + len;
     const char *p1 = p;
-    VALUE str = rb_str_buf_new(len);
     rb_encoding *enc = rb_utf8_encoding();
-    rb_enc_associate(str, enc);
+    VALUE str = rb_utf8_str_new(ptr, len);
+    if (RB_LIKELY(rb_enc_str_coderange(str) == ENC_CODERANGE_7BIT)) {
+        return str;
+    }
+    else {
+        str = rb_str_buf_new(len);
+        rb_enc_associate(str, enc);
+    }
 
     while (p < e) {
         int l, c;