Just as the standard library's PartialEq is implemented for reference types,
the ApproxEq trait should be implemented on them as well. This is mostly an
ergonomic improvement for certain testing situations, where a method yields a
reference. For non-copy types it allows using the assert_approx_… macros, which
would otherwise not be possible.
These macros yield readable error messages as test assertions for ApproxEq
types. They can be invoked as:
assert_approx_eq!(a, b);
assert_approx_eq_eps!(a, b, eps);
Fixes#40.
