Release tracking PR: v0.12.0

[tcharding]

In preparation for release add a changelog entry and bump the version.

(Includes a patch up front to upgrade the version of upload-artifact and download-artifact actions to fix the fuzz CI job.)

[tcharding]

Do you want #200 to go into this @apoelstra?

[apoelstra]

@tcharding yeah -- I'm actually pretty close to having error correction working but it'll require a few more breaking changes.

In particular I need to add a few more fields to the Checksum trait which will break all users that implement that. This is an annoying but ultimately mechanical fix (you need to use the PrintImpl object and print it then copy/paste the result into your code). This might actually be the only breaking change other than #200, but I'm not sure.

I'm tempted to hold off on this release until correction is done, but if the LDK people really want these latest PRs then I'd like to get #200 in and also I'll PR the new Checksum changes, without the rest of error correction, and I'll try to do the rest without further breakage.

[tcharding]

Converting to draft to give you a minute to work on the error corrections stuff.