Add `value!` et al. interface to NDifferentiableConstraints

[anriseth]

The new NDifferentiableConstraints objects introduced in #38 should also get an API for caching and calling value! et al. This would improve some of the code in IPNewton, and make it easier for others to access the constraints objects.

[anriseth]

Ref #38 (comment)

[longemen3000]

i want to help, any general idea to implement this?