Rework Rx API with FRP concepts

[OlivierBlanvillain]

@olafurpg The two middle commits are the interesting bits :)

For reference, here is the elm documentation for these functions, and here is the elm source for the Mario example.

[OlivierBlanvillain]

@olafurpg I did a bunch more reworks, here are the commits that might interest you:

• Rework scala.xml fork: smaller binaries!

• Modernises Chosen, making heavy use of the cats syntax: shows how you can write everything using the new functional API.

• Update README: Lots of new documentation!

[OlivierBlanvillain]

Thanks for the review!