[Proposal] Bindgen for multiple versions of a given library enabled through feature gating.

Hi

For https://github.com/coreylowman/cudarc there was a need to support multiple versions of cuda C library.

Here cuda 11.4 .. cuda 12.8 for instance. This was done previously by generating a binding for each library, and manually import various files with feature flags.

In this PR: https://github.com/coreylowman/cudarc/pull/366 I implemented something that would take various bindgen created binding files, and fuse them into a single file where every symbol is marked with the feature gating which vastly reduces the amount of code generated in the end.

While writing it, I thought maybe it could become something useful more generally for the bindgen community since I took quite a bit of code from bindgen to write it.

Do you think it's something interesting ?
I could try to look up how to implement if there's enough interest.

For a for visual show:

From

#[cfg(feature = "v1.0.0")]
mod v100{
     pub mysymbol;
}

#[cfg(feature = "v1.1.0")]
mod v110{
     pub mysymbol;
     pub myothersymbol;
}

To

// Feature flag elided since every version has this symbol
pub mysymbol;
#[cfg(any(feature = "v1.0.0"))]
pub myothersymbol;

Start by reviewing the linked cudarc pull request #366 and the bindgen code it reused; no repository file or test is named in this proposal. Define the scope and integration point for combining bindings from multiple library versions, with feature-gated symbols and reduced generated output as the completion criteria.