Heads up: metabuild exists

[Kixunil]

I'd like to notify @romanz and other contributors who may be interested that metabuild exists and is now supported in configure_me_codegen.

I obviously do not propose to include it now. Maybe in five years when it's stable and in Debian. :D I'd just like to ask to check it out and give your thoughts in the linked tracking issue so we can get this stable sooner rather than later. For your convenience I have created an experimental branch of electrs that demonstrates the usage of the feature.

Thanks!

[romanz]

Looks great, the usage is indeed much simpler :)