internal/runtime/maps: don't copy indirect key/elem when growing maps

internal/runtime/maps: don't copy indirect key/elem when growing maps

We can reuse the same indirect storage when growing, so we don't
need an additional allocation.

Change-Id: I57adb406becfbec648188ec66f4bb2e94d4b9cab
Reviewed-on: https://go-review.googlesource.com/c/go/+/625902
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>