Stabilize raw::Slice

[mahkoh]

Stabilize raw::Slice and the layout of slices.

Knowing the layout of slices is necessary for efficient ffi code.

[Stebalien]

Triage: raw::Slice has been deprecated in favor of slice::from_raw_parts, [T]::len, and [T]::as_ptr. See discussion on rust-lang/rust#27751 and PR rust-lang/rust#32804

/cc @alexcrichton

[alexcrichton]

Indeed!