Warn when using slices which are unbounded below

[nrc]

Because they might be removed (see RFC PR 498 for discussion). If we do decide to remove these slices, it would be nice to give users some warning, but if we wait for the RFC to be resolved, there will not be much time to do so. Therefore it might be a good idea to warn now that changes might happen (especially since implementation of the warning should be easy).

@aturon what do you think?

[nrc]

Discussed on irc: the slicing syntax is feature gated and all uses of slicing will break due to the removal of the implicit borrow. Plus fixing this is easy - just add 0. So, no need for a warning.