Define a wasi/wasm Runtime object

To do a full port, we'll need an implementation of Runtime with support for running in a wasi and/or wasm environment.

This might be possible based on future tokio/async_std work, but there's a way to go; see #20 (closed) for discussion.

It might also be necessary to define a new Runtime from scratch based on WasmRuntime or something.

See also #343 for issues surrounding lacks of arbitrary TCP sockets.