Support exporting Rust enums to Python (old school / without data)

[davidhewitt]

Following suggestion this is a subset of #417 to focus just on exporting Rust enums without data to Python. These should be fairly straightforward to support - we could borrow inspiration from C++'s boost.Python and pybind11. We might also want compatibility with the Python enum library.

[programmerjake]

I have some messy code that you can use as a reference (originally mentioned in #417): https://salsa.debian.org/Kazan-team/simple-soft-float/-/blob/e19291a0b8eb17e9b52c09b8e670bf0fe3244989/src/python_macros.rs

[davidhewitt]

I'm nominating this for 0.12.

[gilescope]

Jake's code looks like a good starting place. Has anyone had a go at it? Happy to test out PRs. I could have a crack at it myself but I think the PR would need a fair bit of hand holding...

[davidhewitt]

I haven't had a go yet, but I plan to have a go soon unless someone else claims this issue sooner!

[stillinbeta]

I made an attempt (#1045), but I got frustrated by the amount of boilerplate and traits required.

[davidhewitt]

As we've got a fair bit to release in 0.12 I'm going to move this to the 0.13 milestone