Ensure Data Types have a "pure" inheritance tree

[dstufft]

Data Types and "Normal" interfaces need to have distinct inheritance trees for the Python generated code to function. IOW, a data type can't inherent from a non data type, and a non data type can't inherit from a data type.

#271 should solve the case where a data type cannot inherit from a non data type, however it's still the case that a non data type can inherit from a data type. IOW this currently is accepted:

export interface SomeProps {
    foo: str
}

export interface ISomething extends SomeProps {
    bar(): void;
}

However, that could would cause an error in Python, because a "normal" interface cannot inherit from a data type, so we should reject that in the JSII compiler.

[rix0rrr]

This should already be the case. I'm fairly sure I added checks to this effect. No?

[rix0rrr]

Can't seem to find a test for it though ;(

[dstufft]

My memory is that the checks only existed for things that start with I, but that things that don't start with I could still exhibit this problem.

[RomainMuller]

Existing check (jsii already rejects):

• An interface extending another one which is not a struct, cannot be a struct.

Missing checks (jsii will happily proceed):

• A class may not implement a struct interface
• A regular interface may not implement a struct