internal/runtime/maps: avoid table lookup on most Iter.Next calls

internal/runtime/maps: avoid table lookup on most Iter.Next calls

Speeds up iteration by about 3%.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I3406376fb8db87306d52e665fcee1f33cf610f24
Reviewed-on: https://go-review.googlesource.com/c/go/+/621115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>