internal/ui/termstatus: Optimize and publish Truncate

name               old time/op  new time/op  delta
TruncateASCII-8     347ns ± 1%    69ns ± 1%  -80.02%  (p=0.000 n=9+10)
TruncateUnicode-8   447ns ± 3%   348ns ± 1%  -22.04%  (p=0.000 n=10+10)

internal/ui/termstatus/status_test.go

@@ -19,13 +19,14 @@ func TestTruncate(t *testing.T) {
 		{"foo", 0, ""},
 		{"foo", -1, ""},
 		{"Löwen", 4, "Löwe"},
-		{"あああああああああ/data", 10, "あああああ"},
-		{"あああああああああ/data", 11, "あああああ"},
+		{"あああああ/data", 7, "あああ"},
+		{"あああああ/data", 10, "あああああ"},
+		{"あああああ/data", 11, "あああああ/"},
 	}
 
 	for _, test := range tests {
 		t.Run("", func(t *testing.T) {
-			out := truncate(test.input, test.width)
+			out := Truncate(test.input, test.width)
 			if out != test.output {
 				t.Fatalf("wrong output for input %v, width %d: want %q, got %q",
 					test.input, test.width, test.output, out)
@@ -33,3 +34,26 @@ func TestTruncate(t *testing.T) {
 		})
 	}
 }
+
+func benchmarkTruncate(b *testing.B, s string, w int) {
+	for i := 0; i < b.N; i++ {
+		Truncate(s, w)
+	}
+}
+
+func BenchmarkTruncateASCII(b *testing.B) {
+	s := "This is an ASCII-only status message...\r\n"
+	benchmarkTruncate(b, s, len(s)-1)
+}
+
+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 {
+		w++
+		if wideRune(r) {
+			w++
+		}
+	}
+	benchmarkTruncate(b, s, w-1)
+}