Migrate rex to this repository

[mheinzel]

See https://github.com/zinfra/backend-issues/issues/1264.

I have not tried running it yet, but it compiles with the current snapshot.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

✅ mheinzel
✅ jschaul
❌ kim
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[jschaul]

some of those targets don't exist anymore?

[mheinzel]

Yeah, and a few more things need fixing.

• there's an include that doesn't work anymore
• we should probably stack install into dist as with the other tools

I'm not quite sure how rex is supposed to be packaged/used, so maybe someone else could take care of these outstanding issues with the Makefile?

[mheinzel]

I fixed all of the issues except for the Makefile. Once that's done, this PR should be ready.

2 out of 3 committers have signed the CLA.

Kim worked for Wire when writing this code, so we can go ahead without the CLA.

[jschaul]

I added two commits here to put the Makefile in order, add a README, and also to compile a static version based on the current GHC/alpine docker image.

[jschaul]

@akshaymankar any further concerns?

[akshaymankar]

Looks good now.