Support for setting config in shader's Cargo.toml

Values are set in the TOML section: `[package.metadata.rust-gpu.*]`.
So now the priority, from least to most, is:
* Workspace metadata
* Shader crate's metadata
* CLI args

Fixes #15

Notable changes:
* All `clap` args for the `build` and `install` subcommands are now
  defined in the `spirv-builder-cli` crate. This makes passing all the
  config between `cargo gpu` and `spirv-builder-cli` trivial as `serde`
  now does all the heavy lifting of creating and reading the JSON
  arguments.
* There is no longer a `toml` subcommand. The shader crate's `Cargo.toml`s
  are always read and their configs used as the base for CLI args to
  override.

TODO:
* [ ] Reuse the `get_cargo_toml_as_json()` function to parse the
      shader crate's `spirv-std` dependency. This will simplify the code
      and be more reliable.
* [ ] `cargo gpu show targets`
* [ ] `cargo gpu show capabilities`
* [ ] `cargo gpu show etensions`
* [ ] I'm still a bit confused about the `target` and `shader_target`
      arguments, are they both needed?

Cargo.toml

@@ -4,6 +4,9 @@ members = [
 ]
 
 exclude = [
+		# Can't be included because it depends on a specific Rust toolchain and considering that
+		# the reason it exists is to prevent Rust toolchain requirements from polluting workspaces
+		# then let's just not try to workaround it.
     "crates/spirv-builder-cli",
 
     # This currently needs to be excluded because it depends on a version of `rust-gpu` that
@@ -22,6 +25,7 @@ directories = "5.0.1"
 env_home = "0.1.0"
 env_logger = "0.10"
 http = "1.2.0"
+itertools = "0.14.0"
 log = "0.4"
 relative-path = "1.9.3"
 serde = { version = "1.0.214", features = ["derive"] }
@@ -51,5 +55,3 @@ pub_with_shorthand = { level = "allow", priority = 1 }
 partial_pub_fields = { level = "allow", priority = 1 }
 pattern_type_mismatch = { level = "allow", priority = 1 }
 std_instead_of_alloc = { level = "allow", priority = 1 }
-
-