types/views: add iterators to the three Map view types

Their callers using Range are all kinda clunky feeling. Iterators
should make them more readable.

Updates #12912

Change-Id: I93461eba8e735276fda4a8558a4ae4bfd6c04922
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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