Add an Intersection type

[JelleZijlstra]

I promised in python/typing#213 that I'd add support for Intersection, so I'd better do it. I'd like to get Protocol support (#163) in first, though, because most realistic use cases for Intersection involve protocols. Here's a sketch of how the implementation could work:

• Add pyanalyze.extensions.Intersection, which would work similar to Union at runtime. Intersection[int, str] means a value which is both an int and a str. We should also support int & str in string annotations, similar to the existing support for int | str.
• Add a new IntersectionValue to value.py and handle it in lots of places. For example:
  • IntersectionValue.can_assign(val) would check that val is assignable to all members of the intersection
  • val.can_assign(intersectionValue) would succeed if any member of the intersection is assignable to val
  • Getting an attribute on an IntersectionValue should return something if any member of the intersection has the attribute
    • What if they have conflicting values? I guess we can return another Intersection
• Find ways to use Intersection internally. For example, various narrowing operations could return Intersection objects.