Upgrade custom rpc method evm_mine to return the new block

[nicholasjpaterno]

Currently, evm_mine returns 0x0 which is arguably of very little use. This enhancement would reduce the need for users to query for the newly mined block.

[amandesai01]

I am willing to create a PR to close this issue. Let me know if I can.

[davidmurdoch]

@amandesai01 so @MicaiahReid is doing some major work in this area right now so it should wait until that is worked out; evm_mine doesn't have a reliable way of determining which block(s) it actually mined (in some edge cases) right now, so we're fixing it. Once that is complete it should be trivial to implement the feature above.