feature: core.typed annotations

[lvh]

Would this be welcomed if I started contributing them?

[michaelklishin]

If this will make Cheshire depend on core.typed, then I'd vote against it. If this can be made entirely optional, why not.

[dakrone]

I agree with Michael, if optional, yeah, if it added the dependency then probably not.

[lvh]

Would it be okay if it's a dev-time only dependency? core.typed is just a compile-time checker; it shouldn't show up e.g. production uberjars.

[dakrone]

@lvh I think a dev-time only dep is okay with me