Experimental predicate wrappers

[oscbyspro]

I'm adding some property wrappers, for internal use only (#98). The idea is to move preconditions out of method bodies, resulting in fewer "assuming(_:)" signatures. Additionally, I can use them to branch algorithms through type overloading.

- Zero
- NonZero
- ZeroOrLess
- ZeroOrMore
- LessThanZero
- MoreThanZero
- PowerOf2
- NonPowerOf2

/// A predicate that can be referenced by the type system.
protocol _NBKPredicate<Value> { ... }

/// A property wrapper that validates some predicate on creation.
@propertyWrapper struct _NBKGuarantee<Predicate: _NBKPredicate> { }