Replace usage of `Either` with custom made enums.

[ErisianArchitect]

Is your feature request related to a problem? Please describe.
Either can make code harder to read, so I propose that most, if not all, usages of Either in inkwell are replaced with equivalent enums.

Describe possible drawbacks to your solution
We lose the benefit of being able to use Either later on down the road, and it would force us to create new enums.

Additional context
I've already begun working on this PR, I just wanted to make an issue before hand.

[ErisianArchitect]

Oh, and I'd just like to ask in the case that you're alright with these changes: How thoroughly do you want me to replace Either? Because I can make basic enums to replace it, or I can make the enums more equivalent to Either, but that would require a lot more work. I was thinking of making a derive macro for these Either replacement enums so that they can be created more easily. I'd like to hear your thoughts on the matter.