Add a tool that exports the library as a single .rs file

[qryxip]

Like this. I think developing such a tool in this repository is one of the methods that resolve #9.

qryxip@8cbcd09

$ cargo xtask export | xsel -ib # copies to the clipboard
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target/debug/xtask export`
    Expanded /home/ryo/src/github.com/rust-lang-ja/ac-library-rs/src/lib.rs (34003 B)

[qryxip]

With such a tool, we can publish bundled source code directly with GitHub Release or something.