simp
unfold
@Armael: we finally can write `simp [proj]` to unfold the `proj` projection application. Remark: we still need to add similar support for `dsimp`.
See issue #1694. There is an orthogonal issue. `simp` (and consequently `unfold`) cannot be used to reduce projections (e.g., `has_add.add`). This issue has been previously raised by @Armael, but it was not addressed yet.