Use single ContractClassCache

[Oppen]

We currently use two different types for our compiled contract caches:

pub type ContractClassCache = HashMap<ClassHash, ContractClass>;
pub type CasmClassCache = HashMap<ClassHash, CasmContractClass>;

For each of these, CachedState contains an individual field. Both are checked every time we need to refer to a contract.

We also have a CompiledClass enum which has variants for both ContractClass and CasmContractClass, which is actually the one used as return value in the relevant functions.
We can simplify the implementation, as well as get some minor savings, by simply replacing the first two types with:

pub type CompiledClassCache = HashMap<ClassHash, CompiledClass>;

and using a single field containing both kinds of contracts.