GitLab

Accessing the arena index is about to get slightly more complicated.
Abstract this away into a set of functions for going back and forth
between addresses and arena slice indexes.

For #23862.

Change-Id: I0b20e74ef47a07b78ed0cf0a6128afe6f6e40f4b
Reviewed-on: https://go-review.googlesource.com/95496


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>