[PHP7] Static classes

Consumes the PR #911 (ZEND_ACC_FINAL_CLASS removal) and PR #928 (Bison grammar updates for class decoupling)

Static classes are helpful in the case you are wrapping common functions that are static, but the common class itself cannot be instantiated. Currently, PHP developers' only resource is to create a final class with a private constructor. This PR addresses the issue by implementing a similar approach of C#.