Rename Mixin to Style

[rtfeldman]

It's always been pretty weird that backgroundColor returns something called Mixin. My thinking was "these work like Mixins from Sass/Stylus/etc" but really what I want to say is just "these compose" and calling them Mixins seems unnecessary in retrospect.

What I should have said in the first place was "backgroundColor returns a Style, and you can compose Styles to make other Styles"

[mordrax]

w.r.t this PR, the following err now makes alot of sense:

The type annotation for `children` says it is a:

    List Preprocess.Snippet -> Style

But the definition (shown above) is a:

    List Preprocess.Snippet -> Css.Preprocess.Mixin

This was because i included the latest master branch as a local src but didn't know to clean out elm-stuff/packages...