Add explicit truthiness semantics for constant BooleanExpressions #3543

While this PR can use expression negation via __invert__, I think it may be worth supporting Python truthiness for the constant expressions in a follow-up:

class AlwaysTrue:
    def __bool__(self) -> bool:
        return True

class AlwaysFalse:
    def __bool__(self) -> bool:
        return False

That would make patterns like this behave intuitively:

        if not self.visit_starts_with(term, literal):
            return AlwaysTrue()

When adding this, we should also be careful that Python truthiness is for control flow, not expression construction: not expr returns a Python bool, while ~expr returns a negated BooleanExpression. It would probably be safest for non-constant BooleanExpression.__bool__ to raise, with only AlwaysTrue and AlwaysFalse overriding it. The follow-up should also audit optional-expression checks like expr or AlwaysTrue() and REST residual/filter handling so explicit AlwaysFalse() values are not treated as missing.

Originally posted by @kevinjqliu in #3503 (comment)