Add gas price factor.

[adlerjohn]

Fixes #342

Adds a new consensus constant, GAS_PRICE_FACTOR (suggested to be set to 1 billion), that allows gas price to be provided with a precision of 1/GAS_PRICE_FACTOR of the base asset's unit.

Gas used is rounded up to the nearest unit, and gas refunded in rounded down. Note gas and byte costs are accounted for separately. Therefore, the minimum non-zero used balance is 2 coins: one for gas and one for bytes.

E.g. if the base asset is ETH at 9 decimals, then its units are gwei. This change allows gas prices to be specified in wei. If a tx costs less than 1 gwei of gas price * gas limit / GAS_PRICE_FACTOR, then it rounds up to costing 1 gwei.

Some considerations around parameter choice: a script that uses 10M gas can have a gas price of ~100B before you have to start worrying about overflow. Assuming ETH as the base asset:

1. With a factor of 1B, the maximum gas price is ~100 gwei. In the long run, this doesn't leave much room for a robust fee market.
2. With a factor of 1M, the maximum gas price is ~10,000 gwei. Lots of breathing room.
3. With a factor of 1,000, the gas price is 0.001 gwei. This might not be cheap enough. Transactions may still cost on the order of cents even at the lowest gas price.

I'm inclined towards a factor of 1M from the above, but would like to hear what @Voxelot @vlopes11 and @rakita think.

Implementation:

• fuel-tx: Add gas price factor consensus parameter fuel-tx#141
• fuel-vm: Add gas factor to gas cost and refund calculation fuel-vm#148
• fuel-core: add gas price factor fuel-core#412

[vlopes11]

I think its a good call to use the same ratio as gwei since also the community is very familiar with that. Since this won't impact the gas calculation itself but only the assets manipulation, I see this as an improvement without potential problems/concerns.

Recommended steps to implement:

• Implement the constant as consensus parameter in fuel-tx
• Perform the conversion in transaction initialization/post-execution in fuel-vm

[vlopes11]

LGTM