Implement tracking variable values by location

[MikhailArkhipov]

For better intellisense we need to track variable assignment locations so in

x = 1
x. # yields completions for int
x = 's'
x. # yields completions for str

see also #668 for isinstance so

from typing import Union, List

class Foo: pass
class Bar: pass

FB = Union[Foo, List[Bar]]

def do(fb: FB):
    if isinstance(fb, list):
        fb. # completions for list
        return fb[0]
    else:
        fb. # completions for Foo