Update for language changes
Update to latest Rust.
Fix build flags so rust-css will compile.
Update tests for latest Rusts.
Work around compiler bug.
Fix relative font sizes.

The `ComputeFontSizeCb` callback is supposed to return absolute font size, but
returned relative font sizes in many cases.
Derive Eq on all values
color.rs: add colors #1
color.rs: add colors #2(function)
color.rs: add colors #3(test cases)