TrinaryLogic: detect logic errors

[staabm]

allows detecting of dead code like

if ($flagState->yes()) {
	if ($flagState->no()) {
	}
}

[ondrejmirtes]

Oh nice! Thank you.

[staabm]

I think we could narrow the types even more, but atm phpdoc cannot express it well enough I guess.

e.g.

TrinaryLogic->createFromBoolean() can only return TrinaryLogic::YES, TrinaryLogic::NO
TrinaryLogic->createYes() can only return TrinaryLogic::YES
TrinaryLogic->createNo() can only return TrinaryLogic::NO
TrinaryLogic->createMaybe() can only return TrinaryLogic::MAYBE
TrinaryLogic->negate() will leave MAYBE untouched, but invert YES/NO

and conditional types like
when $trinary->yes() is false, this means either $trinary->no()|$trinary->maybe() will be true

might be easier to express in case we can turn TrinaryLogic into a real enum so we could subtract enum-cases