Use atomic wait/notify on RXTAIL in KEY

wipforth.wat

@@ -628,15 +628,24 @@
  ;; Serial I/O
 
  (func $key (local $head i32)
+       global.get $RXHEAD
+       i32.atomic.load8_u
+       local.tee $head
+
        ;; Wait for RXBUF to be non-empty
-       loop $wait
-         global.get $RXHEAD
-         i32.atomic.load8_u
-         local.tee $head
+       loop $wait (param i32)
          global.get $RXTAIL
          i32.atomic.load8_u
          i32.eq
-         br_if $wait
+		 if
+		   global.get $RXTAIL
+		   local.get $head
+		   i64.const -1
+		   memory.atomic.wait32
+
+		   local.get $head
+		   br $wait
+		 end
        end
 
        ;; Read byte at head position