RFC: codify unstable api

[ratmice]

This is an RFC for something that I thought of in the #428 pull request, which codifies the notion of unstable traits and function calls.

Using methods similar to those described by the matrix at, using features to selectively lift the restrictions:
https://predr.ag/blog/definitive-guide-to-sealed-traits-in-rust/

It migrates the couple of methods that are marked unstable in the documentation to take token values indicating their instability.

There are 2 variations of these,

1. InternalPublicApi, that are internal, but public by default (used in generated code).
2. UnstableApi, require a feature to enable their visibility so that functions requiring the unstable token can be called.

The UnstableTrait function/feature is currently unused, but could be useful for #428.

[ltratt]

I think I like it, but I need to get my head around a couple of the implications -- I'll try and get back to you on this soon!

[ltratt]

Does the "key" suffix have a meaning? Should it just be unstable_api?

[ratmice]

No not really, I was mostly thinking of the value of UnstableApi as a key to unlocking unstable_api, and that this feature basically just makes that value public.

the _ prefix does have effect though, in that I believe it hides the feature from cargo/docs.rs

[ltratt]

Oops, yes, I meant _unstable_api (I assumed that _ was a Python-esque convention, but if it's actually noticed by cargo doc even better!).

[ratmice]

Should be fixed in 505f05c, and did some documentation refinement.

[ltratt]

Please squash.

[ratmice]

Squashed.