`eth_estimateGas` result may not be enough

Gas estimation is tricky.

1. A transactions `gasUsed` amount is not the same as the amount of gas required for a transaction.
2. Gas refunds are deducted from the gasUsed amount, but the refunded amount must be available when running the transactions
3. For some OPCODES (like `CALL`) 1/64th of the current gasLeft is withheld from the CALL. This has a strange side effect that can cause the `CALL` (et al) opcode's internal context to run out of gas, even if the transaction itself DOES have enough gas (i.e., `gasLeft - gasUsed >= 0`).
4. Since istanbul, some opcodes, under certain circumstances, actually require more gas to run than they actually spend.
5. The `gasLimit` of a transaction can cause the transaction's `gasUsed` as well as the _required_ gas to vary.

@nicholasjpaterno, did I miss any other tricky cases?

Take a look at ganache-core's gas estimation algorithm for implementation details.