smite: add is_standard_shutdown_script helper

[ekzyis]

Depends on #192.

From the commit message:

BOLT-02 specifies sender requirements for shutdown scripts. They must be
witness v0 (P2WPKH, P2WSH) or following features must be negotiated:

• option_shutdown_anysegwit: witness v1-v16 with a 2..=40 byte program
• option_simple_close: OP_RETURN with a single data push of 6..=80
  bytes

Legacy scripts (P2PKH, P2SH) may be accepted for backward compatibility.

This applies to the shutdown and closing_complete messages, and the
upfront_shutdown_script TLV in the open_channel, open_channel2,
accept_channel and accept_channel2 messages.

This commit adds a helper to catch targets that don't comply with the
spec.

As per the note I added to the code, I'm not sure if the fuzzer should also reject legacy scripts, since a target must not send them. update: decided to reject them, see discussion

I haven't wired this into existing code or #163 yet, but I thought the introduction of the helper might be worthwile to review itself, especially considering the question wrt legacy scripts.

[NishantBansal2003]

Thanks! I was about to add this as a follow-up to #185, but it looks like I don’t have to now

[NishantBansal2003]

Mostly, it looks good. Just a few small comments. I'm not sure how this follows the merge with #192, but if it's merged before, I'll add the updated feature check there

[ekzyis]

I'm not sure how this follows the merge with #192

Putting this into draft until #192 is merged, I rebased this and I addressed the feedback

[ekzyis]

I rebased this PR on top of #192 since I don't expect it to change much. I didn't address all feedback yet, see #186 (comment) and TODO in the code + commit message.

---

Update: rebased again on top of #192 (ACK 40f7aa8)

[ekzyis]

TODO:

• support legacy shutdown scripts for open_channel sent to target (smite: add is_standard_shutdown_script helper #186 (comment))

[morehouse]

Great work! Needs a rebase now that #192 is merged. Also would be good to include the additions from #209 before merging this.

[ekzyis]

c62c850: rebased on master (87ebab6), added is_acceptable_shutdown_script from #192, and updated commit message. I think is_acceptable_shutdown_script is simple and enough for our needs; no need for enum Side like we did for commitments.

f024116: fixed wrong usage of backticks in is_standard_shutdown_script documentation

[ekzyis]

ab9cbad: rebased again on master (d0b2679) + updated commit message

With the new tests, I noticed I could define helpers for p2pkh, p2sh, etc., so there's less to read per test now.