Use slice syntax instead of slice_to, etc.
Put slicing syntax behind a feature gate.

[breaking-change]

If you are using slicing syntax you will need to add #![feature(slicing_syntax)] to your crate.
Rename slice::Slice
Rename slicing methods
Reinstate AsSlice impls for Option and Result
auto merge of #17807 : nick29581/rust/slice6, r=aturon

r? @aturon