CSS: rexport types and variants for computed values.
Add parsing a stylesheet form an iterator, a style attr form a string.
Add supported border styles to the CSS parser.
Remove @pointers from Stylesheet to make it Sendable

… at the cost some extra copying.

This fixes #1081.
CSS: Add a ComputedValues::resolve_color() helper, for currentColor.
CSS: Make sure computed values implement Eq and Clone.
auto merge of #1087 : SimonSapin/servo/newnewcss, r=kmcallister

This is still not the big switch yet, but preparatory changes contained to the 'style' crate.

The current status of the actual port can be seen here: https://github.com/SimonSapin/servo/compare/newnewcss...break-all-the-things (moving as I push and rebase stuff.)