macros: rename crates for consistency

[davidhewitt]

This PR is a simple rename of pyo3cls to pyo3-macros and pyo3-derive-backend to pyo3-macros-backend.

I did this for a couple reasons:

• I think the original names are not very clear.
• The name pyo3-macros matches nicely with the macros feature.

[birkenfeld]

Oh yes please, this is much better.

[konstin]

For historical context, I initially named it pyo3-derive-backend because I wanted to rename pyo3cls to pyo3-derive, but then didn't follow through because I realized you can't rename crates on crates.io

[davidhewitt]

Mmm I think I can manually release 0.12.5 versions of the old names, with READMEs saying to use the new ones.

I chose macros / macros-backend over derive / derive-backend because it's not just #[derive()] macros we provide.

[kngwyu]

I agree that the names are confusing (especially pyo3cls) and renaming is acceptable if we note that in README or so.

[davidhewitt]

👍 I propose I merge this now and immediately release these crates with versions 0.13.0-alpha.1, so that they are available. I will also push 0.12.5 versions of pyo3cls and pyo3-derive-backend which note the change of name.

Then when 0.13 release comes the main pyo3 crate will be published to use these new crates.

[davidhewitt]

@kngwyu unless you say otherwise I plan to merge this tomorrow.

[kngwyu]

Sure, please.

[davidhewitt]

That's done, and all crates updated on crates.io:

https://crates.io/crates/pyo3cls
https://crates.io/crates/pyo3-derive-backend
https://crates.io/crates/pyo3-macros
https://crates.io/crates/pyo3-macros-backend