`validatable` extender forces pureComputed evaluation

I think most library users would expect that pureComputed is computed lazily, that is only when it's value is accessed, since validity is a pure function of an observable's value, the value is only needed when isValid or error or something like that is called, adding or removing validation from an observable does not require it's value to be accessed.

Demo: https://jsfiddle.net/vjdm45g5/

Start with the linked jsfiddle to reproduce the eager evaluation, then locate the validatable extender and trace why adding or removing validation reads a pureComputed value. Done means validation setup no longer forces evaluation, while isValid and error still evaluate correctly when accessed; add or update coverage if the repository has tests for this behavior.