Adds helper methods for HList

You can now:

• Reverse an HList
• Map over an HList
• Fold over an HList

v0.1.9 release

Makes HList a Semigroup thanks to @tupshin 👍

Whereas Semigroup implementations for Tuples are written via macros for up to 26 elements, the HList Semigroup implementation is unbounded :)