Revert overloaded slice notation being merged with library slicing

This PR reverts https://github.com/rust-lang/rust/pull/17620, which caused a significant regression for slices.

As discussed with @alexcrichton, all of the public-facing changes of the earlier PR need to be rolled back, and it's not clear that we should move the libraries over to this new notation yet anyway (especially given its feature-gated status).

Closes #17710
Not intentionally, but we were trying to revert the commits on master as quickly as possible and didn't notice this until too late. @nick29581 was planning to send in a PR to add back the feature gate right away, not sure if that happened yet.