feature: `reduce-right`

This is a WIP that implements `reduceRight` as a lazy fold over the stream, which should be well-behaved on infinite streams.  It's not *quite* what they're talking about in [fantasyland][], since the accumulator argument is a function that produces the value, rather than the value itself, as in `Array::reduceRight`.  Maybe it could be named to `reduce-right-lazy` or something?

This is also my first livescript contribution, too, so I hope I didn't make too many glaring stylistic faux pas.

Thanks for folktale, this looks awesome :)

[fantasyland]: https://github.com/fantasyland/fantasy-land/issues/7