Bring comparison in line with Ruby Sass.
Remove __hash__ from types.

Never used anywhere, and doesn't seem particularly useful.
Test for null equality; fix null repr.
Minor removal of uncallable types code.
Couple more compass helper tests.
Remove code for cross-type operations Sass disallows.
Rewrite the String type.

Unifies quoted and unquoted strings.  Single quotes are now preserved.

Also moves the result of `type-of` into the type system, hurrah.