since 'restrict' will now create intersections when there is
a complex relationship between the two types, calling it
'intersect' makes a lot more sense.
Adds intersection types as a better way to handle the the case
when restrict cannot structurally intersect two types (e.g. when
you learn within a polymorphic function a variable x of type A
is also an Integer, but we dont know how A relates to Integer).
This allows for non-lossy refinements of type info while typechecking.
