fix: ensure `COINBASE` instruction is correct in `debug_traceTransaction`

[robmcl4]

Previously, the instruction COINBASE was returning the address of the prior block's miner, which resulted in an incorrect debug trace. This corrects it to the miner of the traced transaction's block.

[robmcl4]

Actually, it's also necessary to change the difficulty as well -- or else the DIFFICULTY instruction will run incorrectly. For example, it causes this transaction to debug_traceTransaction incorrectly: 0x4fdc0efb59dbc47759bb23069eb5eec83079068818ade9578f57c13adb2b6290

[davidmurdoch]

Awesome. Thanks for finding these issues and proposing fixes. I'd like to add some tests for this before merging. We might want to update this PR to fix DIFFICULTY as well.