Enable contract mocks for CALL OP-codes in solEVM

[johannbarbie]

When Leap transaction run on Plasma, a virtual state environment is created from the inputs of the transaction:

• For each fungible input an ERC20 mock is created and a balance minted.
• For each non-fungible input an ERC721 mock is created and a token minted.
• A mock for the Plasma bridge is created and the last 100 periods with data are added.

During the execution of a spending condition the CALL and STATICCALL OP-codes are intercepted.

• If the destination of the OP-codes is any other address than the described mocks, the transaction fails.
• If the destination is one of the mocks, selected functions are executed.

To create a virtual state environment for Plasma transaction in the EVM enforcer, CALL and STATICCALL OP-codes need to be intercepted in a similar way.

• for transfer() on a token contract a LOG event should be emitted.
• for balanceOf() on a token contract the input's balance should be returned.
• for getPeriod() on the bridge the call should be forwarded to the actual bridge contract.

[pinkiebell]

Does these mocks going to exist as separate contracts or builtins of the EthereumRuntime?

[pinkiebell]

PlasmaVerifier is the skeleton. Closing this, we can issue a new bounty for more specific functionality