This PR makes `fun_induction` and `fun_cases` (try to) unfold the
function application of interest in the goal. The old behavior can be
enabled with `set_option tactic.fun_induction.unfolding false`. For
`fun_cases` this does not work yet when the function’s result type
depends on one of the arguments, see issue #8296.
This PR makes `try?` use `fun_induction` instead of `induction … using
foo.induct`. It uses the argument-free short-hand `fun_induction foo` if
that is unambiguous. Avoids `expose_names` if not necessary by simply
trying without first.
This PR implements `fun_induction foo`, which is like `fun_induction foo
x y z`, only that it picks the arguments to use from a unique suitable
call to `foo` in the goal.
