Predicates for standard wrappers and pair-like aggregate values.

Concepts
concept	StormByte::Type::Optional
	Exactly `std::optional<U>` for some `U`.

concept	StormByte::Type::Variant
	Instantiation of `std::variant`, after stripping cv and references.

concept	StormByte::Type::VariantHasType
	StormByte::Type::Variant `T` that lists `U` among its alternatives.

concept	StormByte::Type::Pair
	Type with accessible `first` and `second` members.

Predicates for standard wrappers and pair-like aggregate values.

Concepts