Add mechanism to evaluate generic aliases in boolean contexts

[dnwpark]

Generic aliases cannot be properly evaluated in boolean contexts. For example, given:

type IsNotInt[T] = not Is[T, int]
type SetOfNotInt[T] = set[T] if IsNotInt[T] else T

then:

    t = eval_typing(SetOfNotInt[int])
    assert t is int

fails with assert set[int] is int

This is because IsNotInt[T] is evaluated as a boolean directly resulting in True.

This PR adds @bool_special_form which can be used on classes:

class IsNotInt[T]:
    __expr__ = not Is[T, int]

allowing the assertion to succeed.