This PR modifies `intro` to create tactic info localized to each
hypothesis, making it possible to see how `intro` works
variable-by-variable. Additionally:
- The tactic supports `intro rfl` to introduce an equality and
immediately substitute it, like `rintro rfl` (recall: the `rfl` pattern
is like doing `intro h; subst h`). The `rintro` tactic can also now
support `HEq` in `rfl` patterns if `eq_of_heq` applies.
- In `intro (h : t)`, elaboration of `t` is interleaved with unification
with the type of `h`, which prevents default instances from causing
unification to fail.
- Tactics that change types of hypotheses (including `intro (h : t)`,
`delta`, `dsimp`) now update the local instance cache.
In `intro x y z`, tactic info ranges are `intro x`, `y`, and `z`. The
reason for including `intro` with `x` is to make sure the info range is
"monotonic" while adding the first argument to `intro`.
