pkg/strings: add SliceRunes

There was some discussions on slack about this and turns out there is no real solution to slice strings easily in cue.

Added a builtin which works internally on runes to work properly on strings with unicode chars.

Not sure about the name of the builtin.

Closes #424
cuelang/cue#424

GitOrigin-RevId: 55640e7cfdf8878e5a90b386fc43b9650656d1bc
Change-Id: I40293d2e1646d1eee2e9074af9954a4be339de78
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6400
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/builtin_test.go

@@ -394,6 +394,9 @@ func TestBuiltins(t *testing.T) {
 	}, {
 		test("strings", `strings.ByteSlice("Hello", 2, 5)`),
 		`'llo'`,
+	}, {
+		test("strings", `strings.SliceRunes("✓ Hello", 0, 3)`),
+		`"✓ H"`,
 	}, {
 		test("strings", `strings.Runes("Café")`),
 		strings.Replace(fmt.Sprint([]rune{'C', 'a', 'f', 'é'}), " ", ",", -1),

pkg/strings/manual.go

@@ -108,3 +108,13 @@ func ToCamel(s string) string {
 		},
 		s)
 }
+
+// SliceRunes returns a string of the underlying string data from the start index
+// up to but not including the end index.
+func SliceRunes(s string, start, end int) (string, error) {
+	runes := []rune(s)
+	if start < 0 || start > end || end > len(runes) {
+		return "", fmt.Errorf("index out of range")
+	}
+	return string(runes[start:end]), nil
+}