math/rand/v2: rename various functions

math/rand/v2: rename various functions

Int31 -> Int32
Int31n -> Int32N
Int63 -> Int64
Int63n -> Int64N
Intn -> IntN

The 31 and 63 are pedantic and confusing: the functions should
be named for the type they return, same as all the others.

The lower-case n is inconsistent with Go's usual CamelCase
and especially problematic because we plan to add 'func N'.
Capitalize the n.

For #61716.

Change-Id: Idb1a005a82f353677450d47fb612ade7a41fde69
Reviewed-on: https://go-review.googlesource.com/c/go/+/516857
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>