Support for calc expressions. Addresses #55.

[nsaunders]

Description of the change This change adds support for [calc expressions](https://developer.mozilla.org/en-US/docs/Web/CSS/calc()) (see #55). This is pretty much a direct port from Clay, which appears to be the original basis for this library. In the process of implementing this feature, I've also aligned the size data types with Clay, which is obviously a significant breaking change but hopefully a worthwhile one...

For reference, here are the relevant modules in Clay for comparison:

1. src/Size.hs
2. src/Border.hs
3. src/Gradient.hs
4. src/Media.hs

Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [x] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation in the README and/or documentation directory Given that the README is pretty empty already, I'm not sure what I should say about this feature...suggestions welcome!
• [x] Added a test for the contribution (if applicable)

[jamesdbrock]

Thanks for this PR!

[jamesdbrock]

I don't know anything about this library and I'm interested in your comment that Clay appears to be the original basis for this library. If that's true then I think we should add that information to the README for the library, to provide a reference point for new users.

[thomashoneyman]

@nsaunders We're looking for maintainers to help with purescript-css (reviewing PRs and/or contributing). This is a great PR -- would you be interested in being added as a collaborator to this repository?

[nsaunders]

@thomashoneyman thanks for the comment... I'm glad I was able to provide some value here and would be happy to help out more in the future. 👍

[thomashoneyman]

Great! I've added you as a collaborator to this repository with maintainer's access. (cc: @jamesdbrock)