feat: make Set directly JSON-unmarshable

[costela]

Currently, Set implements json.Unmarshaler, but it actually panics if you try to use it like this:

type Foo struct {
  Bar *set.Set[string]
}

var f Foo

json.Unmarshal(x, &f)

This PR makes this work for Set, with the minor price of nil checks.
It also makes the zero *Set[T] type directly usable.

TreeSet and HashSet unfortunately cannot be made to support this without further work. The need for external definitions of CompareFunc/HashFunc mean one cannot derive a usable set from the type parameters alone (see the commented-out tests).

[hashicorp-cla-app]

All committers have signed the CLA.

[shoenig]

LGTM; thanks @costela!