Merge the slice methods from ops::Slice with methods in core::slice

[nrc]

Needs a snapshot so we can use indexing, otherwise there's a bunch of changes to something ugly which will change again post-snapshot.

[nrc]

This entails changing all the uses of slice/slice_from/etc. to using the slicing sugar

[nrc]

This includes removing the offensive _s from the Slice methods

[nrc]

I am working on this