Refactor InvocationStatus data structure in memory to improve ergonomics

[slinkydeveloper]

Due to the fact that the InvocationStatus data structure has the Free variant, it requires often to have these noisy accessors like get_invocation_metadata etc. This also ends up in a lot of boilerplate/additional code we don't need, plus there is some common metadata we store in every status. I propose to simplify this a bit:

struct Invocation {
  // common stuff like invocation target etc we retain across all statuses
  
  status: InvocationStatus
}

enum InvocationStatus {
 // Variants with structs specific to statuses
}

This is only a code refactoring (the data structure we deserialize) and has nothing to do with storage changes.