Add cargo xtask export command

[qryxip]

Closes #34.

One of the advantages is that we can release ac-library.rs as a single file with GitHub Releases.

Instead, the CI will take longer. I don't think we will need this tool soon (though an ABC will be held tomorrow), so I'll keep this PR draft.

$ cargo xtask export | xsel -ib # to the clieboard
   Compiling xtask v0.0.0 (/home/ryo/src/github.com/rust-lang-ja/ac-library-rs/xtask)
    Finished dev [unoptimized + debuginfo] target(s) in 1.67s
     Running `target/debug/xtask export`
    Expanded /home/ryo/src/github.com/rust-lang-ja/ac-library-rs/src/lib.rs (46161 B)

[qryxip]

atcoder/ac-library#20

[qryxip]

atcoder/ac-library is trying to

provide the everything combined header file so that we can use AC Library by just copy-paste it to the top of main.cpp

Why don't we provide such a combined file.