Improve update function

Currently we've had to override the update function for a few of our modules in CS, because we need to add in the `many_to_many` or `one_to_many` associations.

It should be possible to abstract this out into this module, by using
`__MODULE__.__schema__(:associations)` to get all of the associations, then map through that with `__MODULE__.__schema__(:association, assoc)` to get detailed information on each association, including whether it's a `many_to_many`, `one_to_many` etc, then comparing it with the updated attributes and dealing with any updated associations in the appropriate way.