Make cargo-gpu a library, usable from build scripts

[Firestar99]

Requires #75 (and base retarget)

Build script API example:

pub fn main() -> Result<(), Box<dyn std::error::Error>> {
    let shader_crate = PathBuf::from("./shaders");

    // install the toolchain and build the `rustc_codegen_spirv` codegen backend with it
    let backend = cargo_gpu::Install::from_shader_crate(shader_crate.clone()).run()?;

    // build the shader crate
    let builder = SpirvBuilder {
        print_metadata: MetadataPrintout::DependencyOnly,
        spirv_metadata: SpirvMetadata::Full,
        ..backend.to_spirv_builder(shader_crate, "spirv-unknown-vulkan1.2")
    };
    let spv_result = builder.build()?;
    let path_to_spv = spv_result.module.unwrap_single();

    // emit path to wgsl into env var, used in `quad.rs` like this:
    // > include_str!(env!("MY_SHADER_PATH"))
    println!(
        "cargo::rustc-env=MY_SHADER_PATH={}",
        path_to_spv.display()
    );

    // you could also generate some rust source code into the `std::env::var("OUT_DIR")` dir
    // and use `include!(concat!(env!("OUT_DIR"), "/shader_symbols.rs"));` to include it
    Ok(())
}

based on https://github.com/Firestar99/colorbubble/blob/eedd20ecc8155f9be15bc95b0fcd68b5002b94f3/build.rs

closes #67
closes #73