Remove KWExistVerifier

[sharplet]

This is the first step in a possible solution to #621.

Currently there's this very loose contract between subjects, verifiers and matchers. At the moment it's possible to pair any kind of subject with any kind of verifier. One problem with this is that a plain old subject can be added to an async verifier, even though it's not possible for the subject to be re-evaluated multiple times.

This is the first simplification of the set of types conforming to KWVerifying.

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Also, because this is a breaking change, I've created a new 3.0-development branch, which this PR targets. It's going to be helpful to have a place that's safe to put these changes!

[orta]

@sharplet it's worth a re-think about whether you want this in

[sharplet]

I think this is only useful as part of a larger set of changes which I'm not really able to commit to. I don't think it's worth merging at this point.