&rsquo; for writing the contents of a bytevector &rsquo;bv&rsquo; to the port.  It
calls &rsquo;await&rsquo; if it has to wait for output to become available.  The
event loop will not be blocked by this procedure even if only
individual bytes can be written at any one time (although if &rsquo;port&rsquo;
references a socket, it should be non-blocking for this to be
guaranteed).  It is intended to be called in a waitable procedure
invoked by a-sync, and this procedure is implemented using
a-sync-glib-write-w