Update for dmd/druntime HEAD.

* `ModuleInfo` now `immutable`.
* `toHash` is now required by AA if `opEquals` is declared.
I don't think this is necessary any more: https://github.com/D-Programming-Language/druntime/commit/d9df5c6418a3ed0d826aa9ad92eda01013670616. Reverting this will avoid breaking code.