Consider usage of types: attofil vs integer in spec

[mhammersley]

@whyrusleeping , @dignifiedquire
Per spec review conversation on specs #131

[whyrusleeping]

It likely makes sense here to explicitly say which fields are ‘currency units’ and not just regular integers. Even though they will likely be handled exactly the same under the hood, it may be helpful to implementations to distinguish these types

[dignifiedquire]

will be fixed with #352