forth: Change stack return type to slice (#1032)
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 <a3aedf8c87c1af3dfeae5f57ddf6d5ae82ad981f@hotmail.fr>
exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust