[perf] Avoid allocating rune array for ascii string

In the best case (all ascii), this reduces the memory footprint by 60%
and the response time by 15% to 20%. In the worst case (every line has
non-ascii characters), 3 to 4% overhead is observed.

src/util/chars_test.go

@@ -0,0 +1,36 @@
+package util
+
+import "testing"
+
+func TestToCharsNil(t *testing.T) {
+	bs := Chars{bytes: []byte{}}
+	if bs.bytes == nil || bs.runes != nil {
+		t.Error()
+	}
+	rs := RunesToChars([]rune{})
+	if rs.bytes != nil || rs.runes == nil {
+		t.Error()
+	}
+}
+
+func TestToCharsAscii(t *testing.T) {
+	chars := ToChars([]byte("foobar"))
+	if chars.ToString() != "foobar" || chars.runes != nil {
+		t.Error()
+	}
+}
+
+func TestCharsLength(t *testing.T) {
+	chars := ToChars([]byte("\tabc한글  "))
+	if chars.Length() != 8 || chars.TrimLength() != 5 {
+		t.Error()
+	}
+}
+
+func TestCharsToString(t *testing.T) {
+	text := "\tabc한글  "
+	chars := ToChars([]byte(text))
+	if chars.ToString() != text {
+		t.Error()
+	}
+}