add unstable exit-with-code function

Whereas the existing `exit` function only accepts a `result` parameter
(i.e. binary success or failure), this function allows the instance to report
any status code from 0 to 255 to the host, with 0 usually meaning "success" and
other values having their own meaning depending on the context.

Fixes WebAssembly#11

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

wit/exit.wit

@@ -1,4 +1,9 @@
 interface exit {
   /// Exit the current instance and any linked instances.
   exit: func(status: result);
+
+  /// Exit the current instance and any linked instances, reporting the
+  /// specified status code to the host.
+  @unstable(feature = exit-with-code)
+  exit-with-code: func(status-code: u8);
 }