fix SetLocal not taking into account the stack bottom

y'know, when you look for locals relative to the call frame's bottom, but then set locals relative to index 0...
boom.

closes #78

crates/haku/tests/language.rs

@@ -266,3 +266,18 @@ fn system_arithmetic() {
     expect_number("1 + 2 + 3 + 4", 10.0, 0.0001);
     expect_number("(2 * 1) + 1 + (6 / 2) + (10 - 3)", 13.0, 0.0001);
 }
+
+#[test]
+fn issue_78() {
+    let code = r#"
+        f = \_ ->
+            let x = 1
+            x + x
+
+        [
+            f ()
+            f ()
+        ]
+    "#;
+    assert_eq!(eval(code).unwrap(), Value::Ref(RefId::from_u32(2)))
+}