Universal contracts for Python special methods

[pschanely]

"Dunder" methods like __eq__ often have properties that we expect to hold true, universally. CrossHair could help ensure those contracts hold. (by some mechanism TBD) A few random examples:

• eval(x.__repr__()) == x (or the repr matches "<.*>")
• x.__eq__(y) == y.__eq__(x)
• x.__lt__(y) or x.__eq__(y) == x.__lte__(y)
• x.__add__(y) == y.__radd__(x)
• x.__reversed__().__reversed() == x
• bool(x) == bool(x.__len__())
• x.__deepcopy__(memo) == memo[id(x)]