You can only have one instance of a module, so it is really more
a collection of types, functions, etc. that help each other do a
particular job.
Then structs and classes can have multiple objects of each type
so you use them to do most the stuff you run. You can make class
objects in a loop or array, you can make temporary ones,
replacable ones, etc. Modules cannot do any of that.
So in short:
* use a module to group classes and functions
* use a class when you need to subclass it and customize
functionality and/or need several instances
* use a struct for most other cases, it is a generic collection
of data
