Basic isinstance() type inference

[JukkaL]

The type checker should know about isinstance checks. For example, this code should be valid:

def f(x: object) -> None:
    if isinstance(x, str):
        # x has type str here.
        print(x + 'foo')  # ok

Initially only support cases where the first argument is a variable and the 'if' condition only contains an isinstance check. Later, consider extending this to more complex cases.