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_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)
 }