Crate crow_util

The utility crate for the crow_engine.

TODO:

• impl Iterator for Holder<T>.
• allow move for SelfRefHolder<T,U>.

Modules

holder	A module containing a basic holder struct, which is used for immutable access to its elements while still being able to insert new elements.
self_ref	A module containing a self referencing struct with one immutable generic parent and a mutable Holder<T> child which can reference the parent.