internal/bytealg: add CompareString

This function was added to Go many years ago, but is starting to be used
in packages in Go 1.23.

src/internal/bytealg/bytealg.go

@@ -42,6 +42,31 @@ func Compare(a, b []byte) int {
 	}
 }
 
+// This function was copied from the Go 1.23 source tree (with runtime_cmpstring
+// manually inlined).
+func CompareString(a, b string) int {
+	l := len(a)
+	if len(b) < l {
+		l = len(b)
+	}
+	for i := 0; i < l; i++ {
+		c1, c2 := a[i], b[i]
+		if c1 < c2 {
+			return -1
+		}
+		if c1 > c2 {
+			return +1
+		}
+	}
+	if len(a) < len(b) {
+		return -1
+	}
+	if len(a) > len(b) {
+		return +1
+	}
+	return 0
+}
+
 // Count the number of instances of a byte in a slice.
 func Count(b []byte, c byte) int {
 	// Use a simple implementation, as there is no intrinsic that does this like we want.