restructure this multi-crate project to use Cargo's "workspace" feature

this solves an issue in the Nix build, where managing multiple
Cargo.lock files is otherwise tricky. it causes (or fails to fix?) an adjacent issue where
the spirv builder doesn't seem to have everything it needs vendored.

crates/spirv_backend_builder/src/main.rs

@@ -0,0 +1,29 @@
+use spirv_builder::{Capability, MetadataPrintout, SpirvBuilder};
+use std::env;
+use std::error::Error;
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    // based on EmbarkStudios/rust-gpu/examples/runners/wgpu/src/lib.rs:maybe_watch
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let crate_path = [manifest_dir, "../", "spirv_backend"]
+        .iter()
+        .copied()
+        .collect::<PathBuf>();
+    println!("get_shader: constructing builder");
+    let builder = SpirvBuilder::new(crate_path, "spirv-unknown-vulkan1.1")
+    //let builder = SpirvBuilder::new(crate_path, "spirv-unknown-spv1.3")
+        .print_metadata(MetadataPrintout::None)
+        .capability(Capability::Int8)
+        // .capability(Capability::PhysicalStorageBufferAddresses)
+        // .capability(Capability::Addresses)
+        // .capability(Capability::GenericPointer)
+    ;
+
+    println!("get_shader: building");
+    let initial_result = builder.build()?;
+
+    let module_path = initial_result.module.unwrap_single();
+    println!("get_shader: built: {:?}", module_path);
+    Ok(())
+}