Streaming responses

[radu-matei]

Guests currently expect the runtime to finish writing the the entire request before reading it, and it would be very helpful to have support for streaming a response body.

One option would be an additional host function that would use the native HTTP library to read chunked responses, write the chunks in the memory, then populate the pointers representing the chunk start and size.
Then, the guest module would read until the size is not zero.