math/rand/v2: start of new API

math/rand/v2: start of new API

This is the beginning of the math/rand/v2 package from proposal #61716.
Start by copying old API. This CL copies math/rand/* to math/rand/v2
and updates references to math/rand to add v2 throughout.
Later CLs will make the v2 changes.

For #61716.

Change-Id: I1624ccffae3dfa442d4ba2461942decbd076e11b
Reviewed-on: https://go-review.googlesource.com/c/go/+/502495
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>