process code with implicit return statement (#1522)

Bookmarklet for instance implicitedly assumes a "completion value" without using `return`.
The `expression` option now supports such use cases.
Optimisations on IIFEs also enhanced.

fixes #354
fixes #543
fixes #625
fixes #628
fixes #640
closes #1293

README.md

@@ -426,6 +426,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
   such as `console.info` and/or retain side effects from function arguments
   after dropping the function call then use `pure_funcs` instead.
 
+- `expression` -- default `false`.  Pass `true` to preserve completion values
+  from terminal statements without `return`, e.g. in bookmarklets.
+
 - `keep_fargs` -- default `true`.  Prevents the
   compressor from discarding unused function arguments.  You need this
   for code which relies on `Function.length`.