This PR removes unnecessary `simp` call in `simpAppFn` in `cbv` tactic
and updates the usage of `cbv_eval` attribute in
`tests/lean.run/cbv1.lean` to follow the new syntax that does not
require an explicit name of the function for which we are registering
the unfold lemma.
This PR adds an experimental `cbv` tactic that can be invoked from
`conv` mode. The tactic is not suitable for production use and an
appropriate warning is displayed.
