Create new contracts package and hashing logic

[rianhughes]

Users will need to compute things like the class_hash for both casm contracts and sierra contracts. To do this new casm types will need to be defined, which I've put in a new contracts package. The old contracts package has been deleted (the few methods defined there will be move over into the accounts package). We also need to implement new hashing functions to compute the class_hash's for both these contract types. I've created a new hash package to store these types (later the hashing logic from the starknet.go file should probably be moved into the hash package).