The addrs.Set type previously snuck in accidentally as part of the work
to add addrs.UniqueKey and addrs.UniqueKeyer, because without support for
generic types the addrs.Set type was a bit of a safety hazard due to not
being able to enforce particular address types at compile time.
However, with Go 1.18 adding support for type parameters we can now turn
addrs.Set into a generic type over any specific addrs.UniqueKeyer type,
and complement it with an addrs.Map type which supports addrs.UniqueKeyer
keys as a way to encapsulate the handling of maps with UniqueKey keys that
we currently do inline in various other parts of Terraform.
This doesn't yet introduce any callers of these types, but we'll convert
existing users of addrs.UniqueKeyer gradually in subsequent commits.
