Check the type of self when accessing a method via instance

When creating a bound method object in statically typed code, the type checker should complain if the type of the method self argument is not compatible with the receiver. For example, this should be an error:

```
class A:
    def f(o: int) -> None: pass
A().f    # receiver is A, self is int: not compatible
```

The issue #16 is related.
