fx update-rustc-third-party

updates rustc_library and rustc_binary third_party dependencies

usage: fx update-rustc-third-party
Updates third_party/rust_crates based on the contents of
third_party/rust_crates/Cargo.toml

See https://fuchsia.dev/fuchsia-src/development/languages/rust/third_party.md
for more details.

update-rustc-third-party source code