This means we can now use type parameter syntax where appropriate.

This commit also includes an upgrade to the golang.org/x/tools module,
in order to get a newer version of "stringer" that supports the type
parameters syntax.

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.

We introduced the addrs.UniqueKey and addrs.UniqueKeyer mechanics as part
of implementing the ValidateMoves and ApplyMoves functions, as a way to
better encapsulate the solution to the problem that lots of our address
types aren't comparable and so cannot be used directly as map keys.

However, exposing addrs.UniqueKey handling directly in the logic adds
various noise to the algorithms and, in particular, obscures the fact that
MoveResults.Changes and MoveResult.Blocked both have different map key
types.

Here then we'll use the new addrs.Map helper type, which encapsulates the
idea of a map from an addrs.UniqueKeyer type to an arbitrary value type,
using the unique keys as the map keys internally. This does unfortunately
mean that we lose the conventional Go map access syntax and have to use
a method-based API instead, but I (subjectively) think that's an okay
compromise in return for avoiding the need to keep track inline of which
addrs.UniqueKey values correspond with which real addresses.

This is intended as an entirely-mechanical change, with equivalent
behavior to what it replaced. If anything here is doing something
materially different than what it replaced then that's a mistake.