smith: use apollo-compiler types

[lrlna]

For historical reasons, a lot of the apollo-smith codebase is based on the parser. Now that the compiler is stabilising, we can unify the apollo-compiler and apollo-smith GraphQL representations, instead of having to maintain yet another one. Ideally, each type generation by the DocumentBuilder would output apollo-compiler types, rather than the custom apollo-smith type, e.g. this function here returns an apollo-smith representations that we then maintain separately. This can be ast::Arugment instead.