feat!: improve storage slot allocation

[nventuro]

Closes #5736
Closes #8659

We currently allocate storage slots for state variables by assuming they are generic over a type T which implements the Serialize<N> trait, and then provide N slots. This is incorrect for some state variables (notably SharedMutable).

This changes that scheme for one in which state variables implement the Storage<N> trait with whatever value they choose, and that N is the one that gets selected for their allocation. All state variables currently use the N from the serialization of their inner type (until we do #5492), so in practice nothing changes so far (but we become ready to perform that work).

I also added a simple test for the storage allocation scheme, which should be very helpful in the future as we make this more complex down the line. I also cleaned up some small warnings I found along the way, and removed the weird exception for #8659, which apparently was unnecessary - it may just be a renmant from some older version of the codebase.

edit: I tried to make the test a bit more thorough using dynamic lengths and got some unexpected errors, I'm looking into those.

[nventuro]

I opened noir-lang/noir#6681 describing the issue I encountered.

[Thunkar]

LGTM!