See Section "Other goodies" at
https://github.com/leanprover/lean/wiki/Refactoring-structures
This commit also improves the support for projections in the
unifier/matcher.
Now, we consider the extra case-split for projections.
Given a projection `proj`, and the constraint `proj s =?= proj t`, we need to try first `s =?= t` and if it fails, then try to reduce.
This is needed in the standard library because we now have constraints such as:
```
@has_le.le ?A ?s ?a ?b =?= @has_le.le nat nat.has_add x y
```
If we reduce the right hand side, we get the unsolvable constraint
```
@has_le.le ?A ?s ?a ?b =?= nat.le x y
```
Before this change, the constraint was `@le ?A ?s ?a ?b =?= @le nat nat.has_add x y`, and we already perform a case-split in this case.
Moreover, projections were eagerly reduced whenever possible.
The extra case-split generates a performance problem in several tests. For example `fib 8 = 34` was timing out.
I worked around this issue by performing the case-split only when the constraint contains meta-variables.
There are also minor issues. Example. `<` is notation for `has_lt.lt`, but `>` is for `gt`.
@johoelzl I'm using `abstract` tactic because instances are
automatically marked as [reducible], and they will be unfolded when
solving unification constraints. This cannot be avoided since we need to
solve unification constraints such as
int.has_add =?= comm_ring.to_has_add int.comm_ring
The `abstract tac` tactic creates an auxiliary lemma to store the proof
generated by `tac`. If we use `print int.comm_ring` we can see that
the definition is much smaller. The proofs are irrelevant. So, this has
no drawbacks, and gives us a good performance boost.
