perf(tactic/ext): NEVER USE `user_attribute.get_param` (#2674)
Refactor the extensionality attribute a bit so that it avoids `get_param`, which is a performance nightmare because it uses `eval_expr`.
```lean
import all
set_option profiler true
example (f g : bool → bool → set bool) : f = g :=
by ext
-- before: tactic execution took 2.19s
-- after: tactic execution took 33ms
```
