Make `offers::Amount` `Copy` and export it in bindings

Make `offers::Amount` `Copy` and export it in bindings

`Amount` is less than two pointers long, so there's no reason it
shouldn't be `Copy`. Further, because its an enum, bindings can't
map a reference to it in an `Option`. Thus, here, we simply make it
`Copy` and return it in `Option`s rather than a reference to it.