Document core traits

[kyouko-taiga]

We should have a detailed documentation of the core traits like Movable, Copyable, Deinitializable, Regular, etc.

Our documentation should assume as little background as possible to maximize inclusiveness for new users. For example, we should not only refer to the term "total order" but also explain what we mean by that term in the context of concrete code one will write and interact with in Hylo.