add meta.yml and generate boilerplate from templates

[palmskog]

@tchajed here is an attempt at using our templates to generate the boilerplate for the project as much as possible, including (opam-based) CI. The hope is that this could simplify maintenance.

What do you think?

[tchajed]

I don't want to write the README in a YAML file and have it auto-generated. It's the main content of this repository and this duplicates all of it and makes contributing a bit harder. Everything else seems good to generate.

I realize some of the README is easiest to generate from the YAML, like the badges. Maybe we just manually copy those in from the generated file? We could add a dev/README.md with a reminder of how that works.

[palmskog]

OK, I'll remove the README content from meta.yml. We can keep some of the auto-generated badge stuff (in README.md), but as-yet it will have to be manually updated in the future.

[tchajed]

The opam build is a little sketchy because it doesn't use the argument customization in the Makefile. For now I think this is fine (and everything builds even without the customizations) - longer term I think we should make such tricks be in a separate directory with their own build setup. That would also be helpful for showing any examples of complete build setups.