feat: guard 语句 guard x >= 0 else { return -1; }

test/programs/26_guard.l

@@ -0,0 +1,11 @@
+fn abs(x: i64) -> i64 {
+    guard x >= 0 else { return -x; }
+    return x;
+}
+
+fn main() -> i64 {
+    print_i64(abs(10));   // 10
+    print_i64(abs(-5));   // 5
+    print_i64(abs(0));    // 0
+    return 0;
+}