Eliminate redundancy in CSS code

[brson]

The CSS code has many layers of abstraction, dedicated to converting unsafe libcss bindings to Servo's own CSS interface. Adding new features involves modifying a pretty deep stack and takes time.

Some things:

• Selector matching requires callbacks to query the DOM. There are multiple layers to this, the low-level unsafe vtable callbacks (netsurfcss::raw_handler), a second set of vtable-based callbacks that capture the high-level trait instance (netsurfcss::UntypedHandler), the high level netsurfcss::CssSelectHandler trait, and finally the css::select::SelectHandler trait that is the actual interface used by servo.
• netsurfcss and css define their own versions of CSS property values that are very similar and converts between them. Some of these can probably be shared, but..
• netsurfcss and css use different techniques for indicating inherited values. netsurfcss makes inherited values just another variant of each property, like CssBorderWidthInherit, while css defines a CSSValue<T> enum with Inherit and Specified variants.
• The CSSValue type allows values to be refined to types that are guaranteed not to equal Inherit, which is nice from a type-safety perspective, but introduces yet another conversion where CSSValues must be 'unwrapped'. This conversion can probably be eliminated while maintaining the nice type properties using datasort refinements.

[burg]

CSSValue was some of the first code I wrote, so it's ripe for ripping.

[SimonSapin]

This is obsolete now that #1109 and #1112 are landed.