Transfer ownership of the name "xz" and "xz-sys"

Hi @alexcrichton! I'm Hyeon Kim, programmer from South Korea.

5 years ago, I created xz and xz-sys crate. Or more precisely, I just allocated the names and forgot updating. At the time, Rust didn't have LLVM bindings at all. I tried to make my own bindings as needed, but because I ran out of time, I completed only xz-sys part and stopped working on xz to do something more urgent.

And five years passed, completely forgetting that I stopped working on xz. Today I looked up the LLVM bindings again as needed, and found that packages named xz2 and llvm-sys were created.

It's a shame that what I forgot has confused people. There were [people](https://users.rust-lang.org/t/recommended-xz-library-for-rust/24718) who were confused about which of the two libraries to use.

Transfer ownership of the "xz" crate and rename this library to xz seems to be a good way in many ways. What do you think?