Introduce types to represent denominations of the base asset (e.g. a Wei type and a Gwei type)

[MitchTurner]

There is a certain danger in doing conversions between different denominations all over the code. It could lead to hard-to-identify bugs if we have it wrong in a single place. We could avoid this by being explicit about which we are using.

The problem is that if we commit to Gwei and Wei, then it's harder to be an L2 of a different chain from Ethereum.

Maybe that is a future concern and the benefits of having these types is too great to avoid--we could solve for the abstraction later.