Make Effect & EffectMapping a monoid structure

[inamiy]

This PR adds Effect, Mapping, and EffectMapping conforming to Monoid structure (2 instances are combine-able with associativity and has identity) for better composition.

Please note that EffectMapping will now have multiple ways of conforming to Monoid since its internal structure is Input -> State -> (State, Effect)? returning Optional as the architectural choice.

Because of this, static func reduce will now introduce enum ReduceStrategy { case first, all, tryAll } for various reducing strategies.

(Previously, this reducing strategy was assumed to have .first only (picking one of the mapping from array), but this prevents from sibling mappings composition, so having .all is essentially needed to behave just as same behavior as Input -> State -> (State, Effect) which returns Identity(Monad) .