slices: document and test nilness behavior of all functions
This change documents the current nilness behavior of all
functions in the package, and asserts each with a test.
There is no change to behavior, but the postcondition is
strengthened, so this may require a proposal.
Fixes #73604
Fixes #73048
Change-Id: Ieb68e609a1248bd81c8507d3795785622a65f8cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/671996
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>