Rust builds up to twice as fast by emitting metadata early

Emitting metadata early makes building/checking Rust up to twice as fast

Headstart patches rustc and cargo to write early metadata as soon as a crate's interface is checked, letting dependents start compiling before dependencies finish type-checking. On 16 cores, clean builds of 13 real projects are up to 54% faster for cargo check and 42% for cargo build, with no slowdowns. Errors still surface with identical diagnostics and exit status.