ui/termstatus: Optimize Truncate

x/text/width.LookupRune has to re-encode its argument as UTF-8,
while LookupString operates on the UTF-8 directly.
The uint casts get rid of a bounds check.

Benchmark results, with b.ResetTimer introduced first:

name               old time/op  new time/op  delta
TruncateASCII-8    69.7ns ± 1%  55.2ns ± 1%  -20.90%  (p=0.000 n=20+18)
TruncateUnicode-8   350ns ± 1%   171ns ± 1%  -51.05%  (p=0.000 n=20+19)

internal/ui/termstatus/status.go

@@ -302,26 +302,35 @@ func Truncate(s string, w int) string {
 		return s
 	}
 
-	for i, r := range s {
+	for i := uint(0); i < uint(len(s)); {
+		utfsize := uint(1) // UTF-8 encoding size of first rune in s.
 		w--
-		if r > unicode.MaxASCII && wideRune(r) {
-			w--
+
+		if s[i] > unicode.MaxASCII {
+			var wide bool
+			if wide, utfsize = wideRune(s[i:]); wide {
+				w--
+			}
 		}
 
 		if w < 0 {
 			return s[:i]
 		}
+		i += utfsize
 	}
 
 	return s
 }
 
-// Guess whether r would occupy two terminal cells instead of one.
-// This cannot be determined exactly without knowing the terminal font,
-// so we treat all ambigous runes as full-width, i.e., two cells.
-func wideRune(r rune) bool {
-	kind := width.LookupRune(r).Kind()
-	return kind != width.Neutral && kind != width.EastAsianNarrow
+// Guess whether the first rune in s would occupy two terminal cells
+// instead of one. This cannot be determined exactly without knowing
+// the terminal font, so we treat all ambigous runes as full-width,
+// i.e., two cells.
+func wideRune(s string) (wide bool, utfsize uint) {
+	prop, size := width.LookupString(s)
+	kind := prop.Kind()
+	wide = kind != width.Neutral && kind != width.EastAsianNarrow
+	return wide, uint(size)
 }
 
 // SetStatus updates the status lines.

internal/ui/termstatus/status_test.go

@@ -49,11 +49,15 @@ func BenchmarkTruncateASCII(b *testing.B) {
 func BenchmarkTruncateUnicode(b *testing.B) {
 	s := "Hello World or Καλημέρα κόσμε or こんにちは 世界"
 	w := 0
-	for _, r := range s {
+	for i := 0; i < len(s); {
 		w++
-		if wideRune(r) {
+		wide, utfsize := wideRune(s[i:])
+		if wide {
 			w++
 		}
+		i += int(utfsize)
 	}
+	b.ResetTimer()
+
 	benchmarkTruncate(b, s, w-1)
 }