internal/runtime/maps: merge Iter.groupIdx and Iter.slotIdx

internal/runtime/maps: merge Iter.groupIdx and Iter.slotIdx

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Ie21ef0f33f42735eadccd75eeebb3b5e81c2f459
Reviewed-on: https://go-review.googlesource.com/c/go/+/618535
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>