Look into using enum instead of public protocols for finite possible types

[drumnkyle]

There are several cases where we have a protocol that is to be implemented by only a finite number of types provided by the library. In this case, we have been making the protocol public though it doesn't make sense for others to implement this as nothing will work.

Therefore, it may make sense to use an enum with cases for each type and an associated value of each to make sure the public interface does not include these protocols which should be internal.

We should explore this as well as generics for things like NoteCollection and NotesHolder.

Also, keep in mind that generics in a public interface are worse for performance.