Reinstate support for stringifiable non-function handlers.

Note that Node's own native implementation of setImmediate neglects to
support this behavior, so I have shimmed global.setImmediate in
test/tests.js in order to make the new test pass in Node.  We might want
to think about shimming global.setImmediate in setImmediate.js, too, but
@domenic thinks we should wait until someone complains, and I agree.