disable cross-validation of defaults

[minrk]

it causes loads of problems with nontrivial validation/observer combinations, including prematurely validating unused values and infinite recursion.

Given that validation of default values should only have an effect when those default values are initially invalid, there seems to be little benefit to introducing the complexity of validating defaults that may not be used.

With this change, ipywidgets tests pass again.

Closes #363
closes #435

#435 is an attempt to fix the problems without removing cross-validation of defaults. I think it's probably better to not cross-validate defaults, but it does get reported as a bug (#428), so we might give it a try.

[rmorshea]

As mentioned in other threads, I agree that the cross-validation of defaults creates too many issues downstream for it to be a worthwhile feature (though it could be done in a backwards compatible way with @default). Given that, I had also felt that it might be simpler from a user's perspective to have non-validated defaults (i.e. no validation phase for defaults). However that may be too large a change to condone. Certainly for now, a simple reversion is probably the way to go.

[minrk]

@rmorshea thanks!