This PR lets `cases` fail gracefully when the motive has an complex
argument whose type is dependent type on the targets. While the
`induction` tactic can handle this well, `cases` does not. This change
at least gracefully degrades to not instantiating that motive parameter.
See issue #8296 for more details on this issue.
This PR lets `induction` accept eliminator where the motive application
in the conclusion has complex arguments; these are abstracted over using
`kabstract` if possible. This feature will go well with unfolding
induction principles (#8088).
