Remove some junk from List constructor.
Clean up List use in Compass helpers and compiler.
Fix bad var name.
Restore __hash__ on types.
Better parity for default operator behavior.  Also fix some bare bools.
Simplified ColorValue considerably.

- Rendering now follows Ruby's rules: shortest representation when
  compressing, otherwise use the original literal, the name, and hex
  notation in that order.

- The old __str__ fallback is gone.

- The `types` attribute is no longer needed.

- The strange `merge` method is no longer needed.

- Got rid of a couple uses of `to_str`, and made it aware of how to
  render types.