FR: Support bitcode for library mode bindings generation

[cerisier]

When using LTO and especially the -Clinker-plugin-lto option, object files are emitted as LLVM bitcode and not machine code.

Currently, uniffi_bindgen only supports proper machine code and fails to extract metadata from object files that contain LLVM bitcode and fails with a Unknown library format.

uniffi-rs/uniffi_bindgen/src/macro_metadata/extract.rs

Lines 27 to 35 in 305454d

	fn extract_from_bytes(file_data: &[u8]) -> anyhow::Result<Vec<Metadata>> {
	match Object::parse(file_data)? {
	Object::Elf(elf) => extract_from_elf(elf, file_data),
	Object::PE(pe) => extract_from_pe(pe, file_data),
	Object::Mach(mach) => extract_from_mach(mach, file_data),
	Object::Archive(archive) => extract_from_archive(archive, file_data),
	_ => bail!("Unknown library format"),
	}
	}

Since the binary metadata extraction is currently solely based on goblin capabilities, I was wondering:

1. Does it makes sense for uniffi to support bitcode ?
   a. Given that it contains all required metadata as in normal machine code.
2. What would be the way to implement it as I am not sure that the goblin crate would accept bitcode support.