Remove Arc<RwLock> from build model types in edr_solidity

[agostbiro]

In #734 we switched to Arc<RwLock> in build model types instead of Rc<RefCell> in edr_solidity. This was necessary, because the EDR provider holds on to those types now to support hardhat_addCompilationResult, so they need to be Send.

Definition of Done

We don't really need to write to the types held by Arc<RwLock> outside the edr_solidity crate, so ideally we would switch back to Rc<RefCell> internally and then expose build model types without interior mutability. Note that this is difficult to achieve, because there are always at least two references to contract structs after interior mutability is no longer needed (one in the deployed contract and one in the deployment contract).