Reorganize wagon-parser

[Rafaeltheraven]

The way it is currently set up is fine, but various structs and traits were kind of haphazardly put into various modules without much thought of what actually makes sense (an unfortunate result from it being the first big crate). Ideally, at some point we go and clean up exactly what structs/traits should be defined where as well as the exact pub scoping of each element.