[Rust] easy packaging candidate: rustfilt

[Josh Stone]

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!