[Rust] easy packaging candidate: rustfilt

Here is a good candidate for packaging: https://crates.io/crates/rustfilt It's like c++filt, but fully understands Rust's mangling scheme, so it should be useful to have packaged, at least for Rust developers. It only has one crate dependency, rustc-demangle, which has no further dependencies, so it should be easy. Two srpms, and we'll have our first complete demonstration of crate packaging!