undefined is a valid Result

[marisbest2]

It feels very heavy handed to require that Result and Either not allow undefined as a valid value.

Given that javascript lacks a void value you can't return Ok() and so its reasonable that we'd use Ok(undefined).
Similarly, Either should be completely agnostic about the internals of the Left and Right types.

I understand why you went with Some(undefined) => None (though I actually don't love that either) but the other two seem unnecessary.

[Note that I think this is just a matter of removing NonUndefined from the types for Result and Either. I don't think it requires any logic changes, though I may be mistaken]

This change is making it very hard to adopt 0.7 and move away from 0.5.10