Interfaces does not check traits types

I'm experimenting with Interfaces, and I noticed that `@provides` does not check for traits types:

```python
from traits.api import Interface, Array, Float, HasTraits, provides
import traits

traits.has_traits.CHECK_INTERFACES = 2

class MyInterface(Interface):

trait1 = Array(shape=[None,2])
trait2 = Float()

@provides(MyInterface)
class TestClass1(HasTraits):
trait1 = Array(shape=[10,2])
trait2 = Float()

# This I expect to work, and it does.

@provides(MyInterface)
class TestClass2(HasTraits):

trait1 = Array(shape=[10,4])
trait2 = Float()

# This I don't expect to fail, but it does not.

@provides(MyInterface)
class TestClass2(HasTraits):

trait1 = Float()
trait2 = Float()

# This I don't expect to fail either, but it does not.
```

Did I miss something ? What would it take to make traits type compatibility checking work ?

Thanks for this very nice library, by the way.