Renamed `app_cons_eq_inv_{l,r}`, and added derived versions for `prefix/suffix`

Merged Jonas Kastberg requested to merge jihgfee/stdpp:prefix_lemmas into master

Renamed the lemmas app_cons_eq_inv_{l,r}, to elem_of_app_cons_inv_{l,r}, to be more descriptive and consistent with existing lemmas.

Additionally, derived versions for prefix/suffix, as they recall some non-immediate properties of the relations on similar list structures.

Edited by Jonas Kastberg

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