Make `Bound` helpers return `Option<Self>`

[maurolacy]

Now that we are using helpers for building Bounds, let's improve the syntax by making those helpers return Option<Bound> directly.

So that instead of Some(Bound::inclusive(key)) or Some(Bound::exclusive(key)), we can just write Bound::inclusive(key) / Bound::exclusive(key).

[maurolacy]

The biggest problem with this is that returning Option<Bound> will break the way we currently map our way through optional bounds:

cw-plus/contracts/cw1155-base/src/contract.rs

Line 497 in 4ec7985

let start = start_after.as_ref().map(Bound::exclusive);

So, voting for closing this without implementing it.

Please re-open if you see a viable alternative here.