Incoporate ideas from WebAssembly/WASI#240

[sunfishcode]

WebAssembly/WASI#240 proposed several ideas for unifying read/write with read/recv and making the interface more general that would be good to pull in here.

[sunfishcode]

Here's a list of ideas from WebAssembly/WASI#240 and their status in #4:

• End-of-stream is indicated explicitly, though it is now indicated with a bool return value on success, rather than $success_eos errno value, which is cleaner now that we're using witx's expected types.
• There is no fdstat. I/O types fully abstracts away the underlying device.
• There is no need for waitall, as WASI I/O functions may not be interrupted.
• Reading from files and sockets is fully unified.
• WASI I/O excludes the terminal support bits. Terminal support will be a separate feature on top.
• Datagram sockets and out-of-band messages are not currently supported, but may be in scope when we can have streams of arbitrary types.
• Connectionless sockets are not supported. These will likely require different I/O Types.
• Streams vs. arrays: this distinction is now being realized; I/O arrays will be updated soon!
• Peeking is not yet supported. I've opened Peek? #6 to track this.