Add debug versions of assert_relative_eq! and friends

[michael-p]

It would be great to have debug_assert_{relative|ne}_{eq|ne} macros that are only checked in non optimized builds, similar to debug_assert! from the standard library.

Implementation should be straightforward, something like:

macro_rules! debug_assert_relative_eq {
    ($($arg:tt)*) => (if cfg!(debug_assertions) { assert_relative_eq!($($arg)*); })
}