Use OP_66 tag for creating permanently unbound inscriptions

[lifofifoX]

Many people tried to create cursed inscriptions with OP_66 tag early on when the cursed inscriptions became a thing. Majority of the first 10k cursed inscriptions are probably made with OP_66 tag. It was an important lesson for the community to learn.

This proposal is to use OP_66 tag for having inscriptions be permanently unbound. This will not affect any of the existing inscriptions in any way.

It will, however, achieve a few things:

1. Provide an official way for creating unbound inscriptions.
2. Allow people to do something with the sats they likely have locked up somewhere due to their optimism.
3. Formally recognize the lessons learned by the community in the early stages of the protocol's development.

And maybe prevent any funky situation that might happen as a result of OP_66 tag being used for something else in the future.

[casey]

I think this is just a docs change, i.e., explicitly state that tag 66 will never be assigned.

Let's create an UNBOUND_TAG in src/envelope.rs and use that in tests which expect an unrecognized even field. (I think we use 22 currently.)

[lifofifoX]

Fixed in #2906

I was hoping to add --unbound flag to inscribe command. That's why I hadn't gotten around to it. But I'll open a separate PR for that when I get a chance to work on.