include tests and license for crates.io archive

[ignatenkobrain]

one of things that you must ship ASL 2.0 license text within sources, but it is also needed for distro packaging

[dtolnay]

Thanks for flagging this. I published 0.2.1 that includes all the files in the git repo. Here is the contents of the crate: https://docs.rs/crate/itoa/0.2.1/source/