Validate default value upon trait instantiation?

This came up a few times in the past (e.g. #886): Currently default value is not validated upon trait initialization, so one could do this
```
from traits.api import Int, HasTraits

class Foo(HasTraits):
age = Int("ABC")

>>> f = Foo()
>>> f.age
"ABC"
```

This issue is for discussing whether or not we should validate default values for all the traits when the trait is instantiated (and inevitably the "how" if we do).