Pretty much what it says on the tin. The idea here is that two different
`Span` values in memory, which compare not equal, might still be the
same duration. Obviously I knew this when I added the trait impl
originally, and realized it could be a footgun, but I thought its
convenience outweighed the footgun. However, I think #32 pretty
convincingly argues that it's the wrong default.
If this ends up being the wrong decision, we can always add the trait
impl back. In particular, I am worried about this making `Span` a very
annoying-to-use type. Not implementing basic equality is just super
annoying because it's common to want it in tests and other various
things where the footgun isn't really relevant. But at least this way,
we can fix the mistake in the future.
Ref #32