Comparison AST does not round trip properly

Discovered this when working on ipy3 stuff:

x = ast.parse("assert False is (2 is 3)")
exec(compile(x, "", "exec"))

The reason it fails is that it's executed as if it was "assert False is 2 is 3".

Note that the generated AST is correct, it's breaking in the conversion between the Python AST and our internal AST. Python's representation is a Compare expression with a list of ops/comparators while we just chain BinaryExpression and assume that if the Right expression is also a BinaryExpression then we have chained comparisons. When we convert from Python AST we just blindly put the right expression in BinaryExpression, in this case it's a BinaryExpression so it thinks we're chaining comparions.

The easy fix would be to wrap the right expressions in a ParenthesisExpression when reverting the Python AST. A better fix would probably be to add a ComparisonExpression type and use that for comparisons instead of BinaryExpression.

Start with the conversion between Python AST and the internal AST, focusing on Compare, BinaryExpression, and ParenthesisExpression handling. Reproduce the provided assert example and trace how its nested comparison is represented. Done means the expression round trips without being flattened into a chained comparison and executes with the original semantics.