I thought it might be useful for future WASI users to have the WASI tutorial written not only in C but also in Rust.
I'm also happy to keep the tutorial up to date with the current state of WASI target in Rust.
Document that `setjmp`/`longjmp` and C++ exceptions are unsupported, and
update the documentation about the function signature mismatch bug to
reflect that it's now just a warning rather than a fatal error.
