* Add the WASI repo as a submodule.

Also, add the witx filenames to the generated output, and just have
`cargo run` auto-generate the api.h header, rather than using clap.

* Switch witx to a path dependency.

* Add a test.

* Add a test that the generated file is in sync with the generator.

* Enable CI testing with Github Actions.

* Fix the name of the wasi-headers directory.

* Enable submodules.

* Add a diff mechanism to help explain failures.

* Sort the inputs for display.

* More debugging.

* More debugging.

* Add a .gitattributes file forcing text files to be eol=lf.

Most editors these days can deal with eof=lf files, even on Windows, and
this avoids trouble with headers and other generated files differing in
line endings.