Identity monads

[AlexXanderGrib]

• Added identity monad
• Added tests for identity monad

Caveats

• Implementation allows multiple parameters (yes, it's not functional style, but convinient. Example - Proposal: Identity monad #45)

TODO

• Docs

[JSMonk]

Hey. Thank you so much for the PR.
Unfortunately, I've already implemented it. But, without the coolest mechanism of providing extra arguments by map, chain proposed by you.
I want to implement it soon, but in a more general way (not only for Identity).
One more time, thank you so much for your activity ^_^

[Lonli-Lokli]

@JSMonk are you going to implement it?