Demangling for Rust symbols, written in Rust.

This builds the C API (librustc_demangle.so/.a + rustc_demangle.h) from the
rustc-demangle-capi crate, so it requires the system Rust toolchain (cargo).

Note: cargo refreshes the crates.io index on first build to resolve an
optional dependency, so the build needs network access (no crates are
actually downloaded -- the only real dependency is in-tree).