Sliced elements do not inherit pre-existing slices on sub-elements

See:
* https://fshschool.org/FSHOnline/#/share/3PX3AJx
* https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/extending.20and.20extension

When slicing an element (e.g. `agent[user].extension[otherId]`) that contains a sub-element that has already been sliced (e.g. `agent[user].extension[otherId].value[x].extension`), the generated slice of the parent element (`agent[user].extension[otherId]`) will not contain the defined slice of it's sub-element. When slicing an element, SUSHI should (probably?) include slices of sub-elements as well.

Start with the linked FSHOnline example and compare the generated parent slice with the pre-existing slice on its sub-element. Read the discussion in the linked chat thread for the intended slicing behavior. Done means generated slices of a parent element retain the defined slices of its sub-elements.

Written by the indexing model from the issue text.