Use Arc for CompiledClass variants

[Oppen]

Currently services::api::contract_classes::CompiledClass contains two variants:

pub enum CompiledClass {
    Deprecated(Box<ContractClass>),
    Casm(Box<CasmContractClass>),
}

Both currently use Box, we should change them to use std::sync::Arc.
These get copied most of the time between the caches and their users, and are essentially read only.
Thus, it would be much cheaper to share them. This will have low immediate effect on performance but will make future work more feasible.