Investigate verifying fraud proofs on a per module keeper level

[jbowen93]

From Bo

@jskybowen was wondering if it was necessary to have the fraud proof verifier be a full ethermint app. The app plumbing seems unnecessary for proof verification (might be missing something here)?

Seems like we could:

• Use the x/evm keeper directly
• Create a custom KVStore impl that can be instantiated using []StateWitness
• Instantiate an x/evm keeper w/ state witness KVStore
• Iterate over []MsgEthereumTx directly and exec within keeper.EthereumTx(msg)
• Perform same pre/post ISR checks