We have a few functions that predate aws-sdk-go, but they have natural
equivalents in aws-sdk-go. Document them as deprecated, and replace
the implementation with the equivalent in aws-sdk-go to make it obvious
that they are the same.
Instead of N pointers, we were returning N null pointers, followed by the real
thing. It's not clear why we didn't trip on this until now, maybe there is a
new server-side check for empty subnetID strings.
A lot of packages use StringSet, but they don't use anything else from
the util package. Moving StringSet into another package will shrink
their dependency trees significantly.