Refactor controls to use transparent pointers

[chemicstry]

This refactors reference types with lifetimes (i.e. ControlListRef<'a>) into transparent pointers, which can be cast into references.

Important changes:

• Immutable<ControlListRef<'a>> becomes &ControlList
• ControlListRef<'a> becomes &mut ControlList
• ControlList can only exist as a reference so new lists are created with ControlList::new() -> UniquePtr<ControlList>. UniquePtr<T> implements Deref<Target = T>.

Fixes #2

[chemicstry]

I'm merging this since it conflicts a lot with other PRs. We will resolve any problems later if they arise.