WASI stdio (stdin, stdout) API not very useful

### What is the problem this feature will solve? Currently the WASI API only lets you give file descriptors for stdin, stdout and stderr. That's not very useful. In most cases I want to interact with these streams which doesn't seem possible - at least not without a ton of platform specific work - with just file descriptors. ### What is the feature you are proposing to solve the problem? You should be able to pass `ReadableStream` and `WritableStream` instead. You can do this when spawning subprocesses. ### What alternatives have you considered? Slightly ludicrously, spawning a new child process that just starts the WASI module and does nothing else seems to be the most reasonable workaround currently, though I haven't tried it.