remove all key usage from Slate's core

[ianstormtaylor]

Do you want to request a feature or report a bug?

Debt.

What's the current behavior?

Right now we use keys all over the place, because we didn't use to have "paths" as a concept. Often the key usage is a crutch for logic that can be made even simpler and more performant by using paths—although this isn't always true. Either way we need to eliminate keys to for #2495 to be able to remove the instantiation step.

What's the expected behavior?

Remove all of the logic in the core slate package that depends on keys. Everything should use paths instead.

[ianstormtaylor]

Fixed by #3093.