Implement monadic and functorial methods in core

[eliperelman]

https://github.com/fantasyland/fantasy-land produced great specifications for creating functors and monadic functionality. I had originally planned to bake this functionality into the core of fn.js, but had removed it at the time because of the size of scope. I would like to see this functionality make it back in, and I think the fantasy-land specification is a good basis for planned functionality, with the exception that fn.js methods should avoid mutating state on user-land objects, and containing state to monadic objects generated by the library.