Made Data.Container(.Any/.FreeMonad) safe by splitting out fixed points

[MatthewDaggitt]

Fixes #1466

[gallais]

I am perversely tempted to suggested adding a sized version of the
least fixpoint and a guarded version of the greatest one. This way
users can pick their poison (lack of safety but more compositional
termination checking vs. safety but annoying checks).

I can prepare the commits doing that & push it to this PR if you
approve the idea.

[andreasabel]

adding a sized version of the
least fixpoint and a guarded version of the greatest one.

It seems that atm there are different choices of greatest fp for indexed vs. non-indexed:

• indexed: musical M
• non-indexed: sized M

I think it would make sense to have all four variants in each case, as @gallais suggests:

• W sized and non-sized
• M sized and musical

[MatthewDaggitt]

Yup, happy to have all the variants included if you want to update the PR @gallais

[JacquesCarette]

Multiplicity of choice, in this case, seems like a good idea to me as well.

[MatthewDaggitt]

@gallais are you going to have time to look at this today? Otherwise let me know and I'll have a crack at it, as I think we'd probably better make a 2nd release candidate sooner rather than later.

[gallais]

Yep, doing it today during AIM.

[gallais]

This is ready I think. But please merge #1517 first.

[MatthewDaggitt]

So to summarise the comments, I think the most consistent structure would be:
Data.Container(.Indexed).Fixpoint.(Sized/Guarded)?

where the Guarded modules re-export the least fixpoints that live in Data.Container(.Indexed)?