Update a few things in the README

[ci skip]
Move FutureCombinators, FutureCallbacks and FutureFactories inside Future

Mostly for documentation purposes, they need to be public and it looks so ugly when they are at the top level.
Add examples to Future API docs
Improve the Future#on_complete API docs
Improve the Future#value API docs.
Improve the Future API docs
Improve the Future#on_complete API docs
Fix an indentation in Future
Add an example to the Future.all API docs
Add an example to the Future.first API docs
Unify the naming in the Future::Factories API docs examples
Clarify two examples in the Future.reduce API docs
Version bump to v1.2.0