Add double-stack words (2dup, 2drop, 2swap, 2over) and forget

tests/forth/stack.rs

@@ -95,6 +95,46 @@ fn tuck_underflow() {
     expect_error("1 tuck", "stack underflow");
 }
 
+#[test]
+fn dup2() {
+    expect_stack("1 2 2dup", &[int(1), int(2), int(1), int(2)]);
+}
+
+#[test]
+fn dup2_underflow() {
+    expect_error("1 2dup", "stack underflow");
+}
+
+#[test]
+fn drop2() {
+    expect_stack("1 2 3 2drop", &[int(1)]);
+}
+
+#[test]
+fn drop2_underflow() {
+    expect_error("1 2drop", "stack underflow");
+}
+
+#[test]
+fn swap2() {
+    expect_stack("1 2 3 4 2swap", &[int(3), int(4), int(1), int(2)]);
+}
+
+#[test]
+fn swap2_underflow() {
+    expect_error("1 2 3 2swap", "stack underflow");
+}
+
+#[test]
+fn over2() {
+    expect_stack("1 2 3 4 2over", &[int(1), int(2), int(3), int(4), int(1), int(2)]);
+}
+
+#[test]
+fn over2_underflow() {
+    expect_error("1 2 3 2over", "stack underflow");
+}
+
 #[test]
 fn stack_persists() {
     let f = forth();