traits-stubs is missing several definitions

(Reported in another communication channel by @rahulporuri)

> it looks like there are a few objects/type declarations missing from the traits.api module

Notably, `HasStrictTraits`, `Interface` and `provides` are missing, but it's likely that these are not the only ones.

As I understand it, completeness was not an initial goal for the stubs, but the three items above are very widely used.