Properly order our `require`s

This provides a single file which requires all others. This file is only necessary for manuall installers who don't have the autoloads.

I gave this a lot of thought, and I think this is the best solution for us. 

The alternative would be to autoload everything inside `sx.el`, but that would be extra work every time we define something that's autoloadable.

With the solution proposed here, we only need to change `sx-load` each time a new file is added. Which is a lot less work.