smite: add closing_sig codec

[ekzyis]

part of #98 | based on #105

This is basically the same as #105.

[morehouse]

All comments from #105 apply here as well.

We can deduplicate the ClosingTlvs a little more I think.

[ekzyis]

Rebased on master (bf52f5f) after #105 was merged, and addressed all feedback (#105+here) in efd1288.

Decided to put everything into a single commit to avoid adding duplicate code in one commit only to remove it in the next. Lmk if you'd prefer it differently.

[morehouse]

There's several divergences from the closing_complete codec that we can fix for consistency. Other than that it looks good.

[morehouse]

Nit:

Suggested change

	data.extend_from_slice(&[0x22; 0x0a]);
	data.extend_from_slice(&[0x22; 10]);

[ekzyis]

done in 17571c5

[ekzyis]

There's several divergences from the closing_complete codec that we can fix for consistency.

Mhh, I'm not sure how I missed these, but 17571c5 should now have addressed all the feedback. I verified this by comparing the output of git range-diff master efd1288 17571c5 with your comments), and has been rebased on master (f99ac3e).

If nits like the one regarding decimal vs. hexadecimal for the vec! lengths repeat syntax come up often enough, we could run grep -R "; 0x" smite smite-* | wc -l and check in CI that the output is 1. Currently, this is the only line with that syntax:

smite/src/bolt/wire.rs:707:        let oversized = vec![0x00; 0xffff + 1];