Support slicing a KnownLayout type

[joshlf]

Given a &[T], Rust supports creating a new &[T] referring to a subset of the original referent, and also supports splitting into two non-overlapping &[T]s that, together, cover the original referent. To my knowledge, this is not supported for slice DSTs.

We could add APIs for these slicing operations to KnownLayout<PointerMetadata = usize>.

One use case for this design would be to support packet formats with explicit length fields - it would be a possibly lighter-weight way of supporting #1289 without requiring solving the general problem described in that issue.