Specializes the congr lemma generated for the `arg` conv tactic to only
rewrite the chosen argument. This makes it much more likely that the
chosen argument is able to be accessed.
Lets `arg` access the domain and codomain of pi types via `arg 1` and
`arg 2` in more situations. Upstreams `pi_congr` for this from mathlib.
Adds a negative indexing option, where `arg -2` accesses the
second-to-last argument for example, making the behavior of `lhs`
available to `arg`. This works for `enter` as well.
Other improvement: when there is an error in the `enter [...]` tactic,
individual locations get underlined with the error. The tactic info now
also is like `rw`, so you can see the intermediate conv states.
Closes#5871
Adds ability to chain congruence lemmas when a function's arity is less
than the number of supplied arguments. This improves `congr` as well as
all conv tactics implemented using `congr`, like `arg` and `enter`.
(The non-conv `congr` tactic still needs to be fixed.)
Toward #2942.
