Allow stable compilation

[Kixiron]

I'd really like to use oak in Differential Datalog, but unfortunately it has the requirement of stable Rust, allowing the nightly features to be toggled with a Cargo feature or removing them entirely to allow stable compilation would be really amazing

[ptal]

Hello! Thanks for your interest.

It is in nightly because of #![feature(proc_macro_diagnostic, proc_macro_span)].
It'll automatically become stable when these features become stable.
I don't think it would be too hard to make a stable version, but unfortunately I have currently no time to allocate on this project...