Vulkano-shaders struct generation refactor

[Rua]

Changelog:

- Bugs fixed:
  - [#1896](https://github.com/vulkano-rs/vulkano/issues/1896): Vulkano-shaders generates invalid struct definitions when struct field names are stripped out by the compiler.

This refactors the code in Vulkano-shaders that generates struct definitions for Rust, to make it easier to read. It also fixes #1896. There are still some problems with the current code, including #1208 and #1530.

[AustinJ235]

Thanks