Value helpers

It turns out you can only type `Sass::Script::Value::<type>.new` so many times before you want to :cry:. I recently passed that limit and coded this up while sitting in a pool of tears.

This is a refactor to make constructing sass values from ruby extensions more convenient. The API is tailored to how developers work with the Sass values instead of how the parser wants to construct the same objects. This refactor significantly cleaned up our own ruby-based Sass functions.

P.S. Yes, I am aware that I can include `Sass::Script::Value` in order to make object creation less typing, but even without that, the creation patterns for developers differs significantly from that of the parser. And I find a function call to be more aesthetically pleasing in this context than a call to `.new` on a class.