Add 'stream' type

Author: lukewagner

design/mvp/Async.md

@@ -453,6 +453,7 @@ features will be added in future chunks to complete "async" in Preview 3:
   no longer wanted and to please wrap it up promptly
 * allow "tail-calling" a subtask so that the current wasm instance can be torn
   down eagerly
+* allow pipelining `stream.read`/`write` calls/buffers
 * `task.index`+`task.wake`: allow tasks in the same instance to wait on and
   wake each other (async condvar-style)
 * `nonblocking` function type attribute: allow a function to declare in its