all: use iterators in more places instead of Range funcs

And misc cleanup along the way.

Updates #12912

Change-Id: I0cab148b49efc668c6f5cdf09c740b84a713e388
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f