Refactor: create `blockchain` struct that contains a `vm`.

[mpaulucci]

Depends on: #1949

We should create a blockchain struct singleton, that contains a vm struct.

This could be initialized as: let blockchain = Blockchain::new(EVM::REVM), or directly passing the vm object.

Then, when we execute a block, we call blockchain.add_block(), which calls vm.execute_block().

Acceptance criteria:

• should also remove get_evm_backend_or_default() that accesses the global vm object, we should remove code related to the global object.