Add `unstable_ir` feature flag that makes the ir pub

[heesooy]

Fixes #1009

[zhp46]

This would be a fantastic addition to have

[heesooy]

I guess this is reasonable to do, but I don't want to make any change to the ir a breaking change.

Let's call this unstable_ir and document it with no stability guarantees in docs.md (that is, if you use this, you need to pin the library, and are on your own).

With that, this looks reasonable to me, if it simplifies life downstream compared to a fork...

@emilio -- done; renamed to unstable_ir and added a section in the docs. Let me know if you want me to move the docs to a different section, wasn't sure where would be best. Thanks for taking a look!