Unify `Guard` and `PoolGuard` types about sharded-slab

[bIgBV]

@hawkw yeah I'd like to work on this. And the Clear impl for Option looks like a great way to move forward with this.

[hawkw]

In the Slabs case we need the type to be an Option<T> in order to call Option::take.

Since we're not actually returning the taken item in this code path, can't we just use the Clear impl for Option?

sharded-slab/src/clear.rs

Lines 15 to 19 in ffa5c7a

	impl<T> Clear for Option<T> {
	fn clear(&mut self) {
	let _ = self.take();
	}
	}

This should have equivalent behavior to the current implementation. I think it should be fine to have one code path that's used by both guards. It's only Slab::take that requires special-casing.

[hawkw]

@bIgBV Are you interested in working on this? I'd like this to be addressed before we publish a release with the object pool API. If you're busy, I'm happy to take care of it so we can move closer to a release.

[hawkw]

Okay, great. I think this is the last release blocker for the pool.

[hawkw]

Looks like this probably isn't going to be possible with the current state of things (see #33 (comment) and #33 (comment)) so I'm not going to block the release on it.