Enable creating AssetIds from ContactIds and SubIds

[bitzoic]

Currently there is no way to get an AssetId if you have a ContractId and a SubId. This means if you mint an asset from a contract, you do not know the AssetId of what you just minted. Something similar to this needs to be added to the rust SDK:

pub fn get_asset_id(sub_id: Bytes32, contract: ContractId) -> AssetId {
    let mut hasher = Sha256::new();
    hasher.update(*contract);
    hasher.update(*sub_id);
    AssetId::new(*Bytes32::from(<[u8; 32]>::from(hasher.finalize())))
}

Documentation on this should also be included in the AssetId section of the book