* module/language/ecmascript/base.scm (prop-keys): New method, returns
the list of keys of props of this instance.
* module/language/ecmascript/impl.scm: Refactor the global object into a
special kind of module object. Provide a prop-keys implementation for
module objects.
* module/language/ecmascript/compile-ghil.scm (comp): Compile for-in.
* module/language/ecmascript/impl.scm: Reshuffly things, and implement
make-enumerator, a helper for use in for-in statements.
* module/language/ecmascript/parse.scm (parse-ecmascript): Fix parsing of
for (var foo in bar) {}...
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