Type inference for multi-statement initialization

[JukkaL]

Improve type inference by making it possible to infer the type of a variable from multiple (conditional) assignments.

This should be valid:

x = None
if c:
    x = 'a'
# type of x is str

Another example:

if c:
    x = None
else:
    x = 'a'
# type of x is str

Later on, we could extend this to precise typing of None values using union types.

Edit: restricted this issue to multiple initialization.

[JukkaL]

We should not generally infer union types like this, as it would probably be too confusing. This would only be used for None and potentially in cases where there is a non-trivial common supertype, such as in cases like this:

class Animal: ...
class Dog(Animal): ...
class Cat(Animal): ...
if c:
    x = Dog()
else:
    x = Cat() 
# Type of x is Animal? Maybe even Union[Cat, Dog] since Cat and Dog are related, but we don't
# want to generate unions such as Union[int, str], I think, unless x is explicitly declared
# to have a union type.

[JukkaL]

#1099 covers the case of None initializer. Let's not infer union types or common supertypes yet.