This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
"Property testing" is heavily used, tests that generates random cases and asserting properties.
That gives high confidence of round-trip correctness of `SliderPath` conversion, for example.
