Merge generic enum tags

[Benjamin-Davies]

See attached issue.

Resolves #784

[morrisonlevi]

Looks good to me as a total outsider. @emilio, are you the right person to ping these days? You have the most commits last year.

[emilio]

Sorry for the lag here, I haven't been able to take a look lately at cbindgen stuff.

In general the feature makes sense, but instead of the Rc<Cell<>> shenanigans and having to keep the unmangled paths around, it seems it could be implemented more cleanly and efficiently in instantiate_monomorph. At that point, you know you have a generic, and you're going to monomorphize it. It should be feasible to synthesize a tag enum there, and just carry an is_monomorphic: bool or so to avoid generating the tag if needed.

[emilio]

Thanks! Looks much better. Some final requests, and also can you add the new option to the docs?

With that it looks good.

[emilio]

Thanks, if CI is green this looks good. Can you squash your commits in the way you best see fit so that we don't have "Tidy up" commits? Or would you be fine with me squashing before merging?

[mxyns]

hi all, i am interested in getting this PR merged as it would greatly help me in my project 🙂 Is there something I can do to make this happen?
@emilio are you still the person to ping or should is there someone else you can refer us to?