Unify Table* and Collection* components

[jessesquires]

We should be able to unify our table- and collection-specific components into single, generic components using a similar approach to https://github.com/jessesquires/JSQDataSourcesKit.

The model in JSQDataSourcesKit and here are a bit different, but we should be able to do this. The main difference is that JSQDataSourcesKit does not consume the delegate properties — only the dataSource.

Todo (roughly):

• Unify CollectionViewCellViewModel and TableViewCellViewModel --> CellViewModel<T>
• Unify CollectionViewModel and TableViewModel --> CellParentViewModel<T>
• Unify the nested SectionModels
• Unify CollectionViewDataSource and TableViewDataSource --> DataSource<T>

Then, we can implement the UITableViewDataSource and UICollectionViewDataSource methods in constrained extensions on DataSource<T> similar to this:
https://github.com/jessesquires/JSQDataSourcesKit/blob/develop/Source/DataSourceProvider.swift#L64

[jessesquires]

This will be a pretty large change and we need to refine / flesh out existing APIs first.