* wasi-headers: update WASI submodule, handle changes to witx ast
* wasi-headers: restructure lib and exe to be more flexible
just factor out some of the hard-coded stuff
* 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.
