[red-knot] add type narrowing (#11790)
## Summary
Add Constraint nodes to flow graph, and narrow types based on that (only
`is None` and `is not None` narrowing supported for now, to prototype
the structure.)
Also add simplification of zero- and one-element unions and
intersections, and flattening of intersections.
There's a lot more normalization logic needed for unions and
intersections (as is obvious from the inferred type in the added
`narrow_none` test), but this will be non-trivial and I'd rather do it
in a separate PR.
Here's a flowchart diagram for the code in the added `narrow_none` test:
The top branch is for the `if` expression in the initial assignment to
`x`; that `Constraint` node would only affect the type of `flag`, which
we don't care about in this test.
The second branch is for the `if` statement, with `Constraint` node
affecting the type of `x`.
## Test Plan
Added tests.
