Linker-Based Asset Propogation

[DogeDark]

This PR is built off of the great work done in #26. Instead of searching the compiled binary for link sections, we create helper functions for CLIs to act as a linker "intercept" to grab assets directly from unlinked object files.

While being a more ideal solution, it does complicate things:

• CLIs need special handling possibly separate from normal command handling when used as a linker.
• There is currently no sure-fire way to know the CLI is being called as a linker.
• The CLI is run twice meaning that collected asset information is not directly available to long-running tasks such as a serve system. The asset information needs to be provided through temp files, sockets, or other methods of propagation.
• We currently call cargo twice. First for the regular build, second for the linker intercept. This can be omitted with once-stabilized nightly features or more difficult linker-detection systems. If rust-lld becomes the default everywhere, this could become much simpler.
• In my testing, the working directory was changed while cargo and rustc did it's thing. To solve this, we pass the current working directory as a linker arg that is collected when the CLI is called as a linker.

What Is Added:

// A linker_intercept function that returns a `Some()` if it detects linker args. 
// A list of user-provided args are passed through linker args and paths to every object file (rlib, o) is provided.
// Option<(linker_args, object_files)>
manganis_cli_support::linker_intercept(env::args()) -> Option<(Vec<String>, Vec<PathBuf>)>; 

// A helper function to call the current running exec as a linker by calling `cargo rustc` with the passed `args`. 
// Subcommand is the subcommand the current executable should be called with.
manganis_cli_support::start_linker_intercept(subcommand: ToString, args: IntoIterator, linker_args: IntoIterator);

// A helper function to get manganis json strings from a list of paths to object files.
get_json_from_object_files(object_paths: Vec<PathBuf>) -> Vec<String>;

// AssetManifest load function has changed to accept a list of manganis json strings.
AssetManifest::load(json: Vec<String>) -> Self;

// AssetManifest has a new `load_from_objects` function. 
// This just calls `get_json_from_object_files` and `AssetManifest::load()`.
AssetManifest::load_from_objects(object_files: Vec<PathBuf>) -> Self;

Testing
The way a linker is called seems fairly consistent across platforms, but that doesn't mean we missed something!

• Windows (link)
• Linux (cc)
• wasm32-unknown-unknown (rust-lld)
• Mac (cc)

Other platforms?

[DogeDark]

I made some changes, can we have Mac tested again?

cd test-package
cargo build -p manganis-cli-support --example cli
../target/debug/examples/cli build

Testing if the shell command executes successfully. If the example exits without error, it works.

[jkelleyrtp]

Fixes look great - tests pass for me on mac!

MASSIVE THANK YOU! @rambip @DogeDark @ealmloff

This is amazing - a huge win for the Rust asset system. I'm very excited to build more tooling around this, both for dioxus and for other projects also looking to support assets.