Adjust color type to make it more safe #746

The problem

Now that we have PlatformColor (nice implementation btw), I think we should think about color not being a type alias to string anymore (part reversal of this PR). I would like to gradually start to use platform colors in my app, now all colors are strings, and sometimes I do some manipulation on them (like darken/lighten), of course that wouldn't work on platform colors.

Considered solution

So to make colors safe I think we should make them phantom typed.

type t<'a>
external string: string => t<string> = "%identity"
external get: color<'a> => 'a = "%identity"
...
let blue = Color.string("blue")

let blueString = Color.get(blue)

With platform colors being color<PlatformColor.t>.

In Style.t we can accept all colors (we need a lot of type parameters in the external though)

Alternative solution

An opaque type. The type of the color would be lost, so we can't easily have a get or toString that would raise a type error for a platform color. The upside for this is that we need less type parameters.