casting traits inherit from mixin

[rmorshea]

Adds a generic CastingMixin class that all other casting TraitTypes inherit from (i.e. CInt, CFloat, etc.). Upon setting a value, these reconfigured traits first coerce the input value to the proper type, and then pass it to super().validate(...). This mixin also makes new casting traits like CDict and CList more sensible to implement, because the complex base class validation heuristics don't need to be hard coded in their casting versions anymore:

class A(HasTraits):
    cd = CDict()

a = A()
a.cd = [("a", 1)]
# prints: {'a': 1}
print(cd)

[minrk]

This can be defined as the default on the mixin, rather than using getattr with a falllback

[rmorshea]

Working on a more generic solution...