Cleanup

[ghost]

The original goal of this cleanup was to remove all uses of ref and ref mut, but I ended up doing other things too, including some potentially controversial ones:

• Replacing a use of Option::cloned with the more explicit Option::copied, which would limit use of this crate on stable to versions 1.35 and greater. Right now the latest stable version is 1.34, but this crate cannot be used on stable yet anyway, so this is not a breaking change.
• Deriving Debug for three public types in order to replace assert!s with assert_eq!s.
• Removing unnecessary derefs even though they're currently the prevailing style. Another open pull request, #16, would add even more of them, so I'd like to get @Gilnaa's opinion on this.

[Gilnaa]

This looks good, I guess.