forth: Change stack return type to slice (#1032)

Closes #961: returning an owned value for a getter that is used for
tests only is a bit silly, so this changes the return type of the
`Forth::stack` method from `Vec<Value>` to `&[Value]`. It does not
require changing the tests.

Signed-off-by: Paul Mabileau <[email protected]>

Author: PaulDance

exercises/forth/src/lib.rs

@@ -16,7 +16,7 @@ impl Forth {
         unimplemented!()
     }
 
-    pub fn stack(&self) -> Vec<Value> {
+    pub fn stack(&self) -> &[Value] {
         unimplemented!()
     }