Do not return any possibilities when on the foreign stack.

From now on, when the foreign stack is in effect, salve will always
return an empty set of possibilities. Otherwise, it would return
possibilities that do not make sense in while the stack is in effect.
Infer misplaced elements when possible.
Version bump.
Updated the dependencies.
Merge branch 'release/0.20.0'
Merge tag 'v0.20.0' into develop

0.20.0