State Management examples

[KyrylR]

It would be nice to reorganise Simplicity contracts that work with state via Taproot.

Currently, they are just in the contracts repository. I would suggest keeping them under the state_management folder, or something similar.

Also, right now we have four different storage examples:

1. Asset-based storage
2. Taproot storage when we need to store up to 256 bits of data
3. Storage where we have witness::STATE_1, witness::STATE_2, etc. (array-based storage)
4. Storage where we store the root of the Merkle tree, and then use Merkle tree proofs to read/update storage

The bytes32 storage already works, though MT-based and array-based (both Taproot storages) are under review.

Eventually, what we want to have is a structure with documentation, helper functions, and examples to help others understand how to work with it.