Skip rune decoding for ASCII bytes in ToChars

utf8.DecodeRune already fast-paths ASCII, but it is too complex to inline
(cost 201 against a budget of 80), so a mostly-ASCII line pays one call
per byte just to be told the byte is ASCII.

Only the run after the first non-ASCII byte reaches the decode loop, so
the gain depends on where that byte falls.

- 70-rune ASCII line: 145ns -> 42ns in the decode loop
- Ingestion of 1.4M mostly-ASCII paths behind a Hangul prefix, where the
  loop covers the whole line: 377ms -> 233ms
- The same paths with the Hangul at the end, where it covers six bytes:
  207ms -> 196ms
- Break-even sits at ~100% non-ASCII runes: still 1.01x at 95%. Only a
  line holding no ASCII byte at all loses, by ~0.14ns per rune, ~5% of
  the loop

src/util/chars.go

@@ -77,6 +77,13 @@ func ToChars(bytes []byte) Chars {
 		runes[i] = rune(bytes[i])
 	}
 	for i := bytesUntil; i < len(bytes); {
+		// utf8.DecodeRune has an ASCII path of its own, but it is too complex
+		// to inline, so a mostly-ASCII line pays one call per byte for it.
+		if b := bytes[i]; b < utf8.RuneSelf {
+			runes = append(runes, rune(b))
+			i++
+			continue
+		}
 		r, sz := utf8.DecodeRune(bytes[i:])
 		i += sz
 		runes = append(runes, r)