Remove dependency of requests on Session/Element for layering

[tristanlabelle]

There's a conceptual layering in the codebase where the Session and Element structs can be built on top of Request structs, but this is not apparent because Request structs refer to Session/Element types. This refactoring is a step towards making this layering explicit. A follow-up will be to pull the requests in separate files.

• Replace Request dependencies on Session/Element with strings
• Make Request struct properties var instead of let for more swiftiness
• Rename ***Request variables to request since the type is apparent from the context
• Changed all var body properties to computed properties based on fields of the request

[jeffdav]

Love it.