Add `@interface` decorator

[certik]

Currently we define a function interface using:

def complex(x: f64, y: f64) -> c64:
    pass

But that is the same syntax as for an empty function. I think we should add an @interface decorator into ltypes, which would probably raise an exception if the function was ever being called (by CPython). In LPython, that would declare an interface, meaning the function with that name must be provided in global scope at some point before compiling to LLVM.

[Smit-create]

Do you mean something like this?

def interface():
    raise Exception('Unexpected to be called')

@interface
def complex(x: f64, y: f64) -> c64:
    pass

Internally, we will handle lpython when it sees an interface operator, and CPython is not expected to call this function at all.

[certik]

Yes, exactly. (The CPython implementation I think has to be a nested function though.)

[Smit-create]

Okay, I will try to get it implemented.