Support for generics

[ArtiomTr]

This pull request adds support for generics in bindings. The code is pretty rough, but it does the job.

The generics are really handy, keeping in mind that this crate doesn't support macros. I haven't wrote tests for this code, however, it is used to generate rust-csharp bindings for grandine. It contains pretty complicated cases, including:

• Returning generic type from function pointer inside struct:

  https://github.com/ArtiomTr/grandine/blob/72ef37721dbd5340fe4881c474f311ee486d772f/bindings/csharp/Grandine.NethermindPlugin/NativeMethods.g.cs#L293

• Putting generic type inside generic type:

  https://github.com/ArtiomTr/grandine/blob/72ef37721dbd5340fe4881c474f311ee486d772f/bindings/csharp/Grandine.NethermindPlugin/NativeMethods.g.cs#L299

I've also added cargo::warning= prefix to all printed messages, as otherwise they aren't visible when using this crate in build.rs script.

[ArtiomTr]

Initially I thought this would be an easy change, however, looks like C#'s marshalling doesn't work quite good with generics, even with instantiated ones. So probably this change is out of scope for this repo