Add optional gas tracking

[eddyashton]

While not required for the eEVM's original purpose, other users of the eEVM will want gas to be tracked and expended as specified in the white paper.

If added, this should be an optional additional feature - disabled at compile time in applications where it is not needed.

This is currently a low priority, but I expect it will eventually be a blocking requirement for a potential user.

[nagendras176]

I would like to work on this.