Encoding Proposal for Bitcoin Multisig Address as Wallet ID

wallet ID thoughts.txt

Separately, I'm now conflicted on whether the first address should be encoded in some non-address format, so that there is no ambiguity for end-users.

Here's one imperfect solution that would be trivial for any bitcoin wallet developer to implement:
1. Fetch receive address 2,147,483,647 (`2^31 -1`) for the descriptor record, let's call this address X
2. Calculate the double-sha256 digest of X in hex
3. Truncate to something more reasonable, perhaps 160 bits (40 characters in hex)
4. Prepend some identifier for UX, I suggest `MW-` (multisig wallet).

I find code much clearer as a way to communicate, here is what that looks like in python:
```
>>> from hashlib import sha256
>>> RECEIVE_ADDRESSS = "tb1qlrjv2ek09g9aplga83j9mfvelnt6qymen9gd49kpezdz2g5pgwnsfmrucp"
>>> encoded_address = sha256(sha256(RECEIVE_ADDRESSS.encode()).digest()).hexdigest()
>>> print("MW-" + encoded_address[:40])
MW-82bd5553796fe13b8244368d56eed9997893b953
```

Or in bash:
```
$ echo -n MW- && echo -n tb1qlrjv2ek09g9aplga83j9mfvelnt6qymen9gd49kpezdz2g5pgwnsfmrucp | openssl dgst -sha256 -binary | openssl dgst -sha256 | cut -c 10-49
MW-82bd5553796fe13b8244368d56eed9997893b953
```

Bech32 encoding would be even better as it's designed to handle this problem, but then it starts to look like an address to end-users and we're back to square 1.

--

Pros of encoding:
* Using an address as a wallet ID is inherently confusing, in an already confusing process.
* Some users will mistakenly think this is the only address they're supposed to use, and this will encourage address re-use.
* This (gently) encourages poor privacy practices as it links one address to your multisig wallet. Perhaps you have a file backed up to the cloud that lists your assets, and one entry is called `Bitcoin Wallet ID [FIRST RECEIVE ADDRESS]`. An entry called `Bitcoin Multisig Wallet MW-82bd5553796fe13b8244368d56eed9997893b953` leaks no information (unless of course you create a transaction to/from the 2,147,483,647th address).

Pros of just using the first receive address:
* If users do send bitcoin to that address, they can easily spend it. There is no risk of loss.
* Bech32 is already designed to make manual verification as easy as possible.
* It requires very minimal developer effort and has less complexity.
* Privacy conscious users saving data in the cloud could reference their wallet by its bitcoin core checksum. It should be ovbious to even newbies that saving a bitcoin address in the cloud is a privacy concern.