all: rewrite internal/saferio.SliceCap using generics and add func SliceCapWithSize

all: rewrite internal/saferio.SliceCap using generics and add func SliceCapWithSize

Change-Id: I265173bf2722796c4be545c968efef3a1a6f7a7d
GitHub-Last-Rev: 04d95cdd615f906167545f246f707e1440c39374
GitHub-Pull-Request: golang/go#62365
Reviewed-on: https://go-review.googlesource.com/c/go/+/524257
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>