Add support for closures

This proof of concept shows how it would be possible to support closures as members of a thread class - see #67.

```sh
Timm@slate ~/devel/php-src [master]
$ make test TESTS=ext/pthreads
# ...
Number of tests :   53                53
Tests skipped   :    0 (  0.0%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    0 (  0.0%) (  0.0%)
Expected fail   :    0 (  0.0%) (  0.0%)
Tests passed    :   53 (100.0%) (100.0%)
```

*TODO: Pollutes the EG(function_table) like create_function(), which is not nice