vm: fix symbol access

By using the new SetHandler API instead of SetNamedPropertyHandler, we can
intercept symbols now. This forces us to use Maybes and MaybeLocals more,
since this new API does not have a non-maybe variant.

Fixes: https://github.com/nodejs/io.js/issues/884
PR-URL: https://github.com/nodejs/io.js/pull/1773


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>