Improve static safety of repo by using generic types and custom constructors

As per our discussion here: https://github.com/Azure/azure-functions-durable-python/pull/184#pullrequestreview-544266683

Data such as the Entity batch array would be safer to use if registered as their own datatype. This way, we can prevent invalid operations across the codebase.

We should do a scan of the codebase and improve all the cases where custom types could be used to prevent invalid operation.

Start with the discussion in pull request 184, especially the linked review, and inspect the Entity batch array example. Then scan the codebase for other places where custom types or constructors could prevent invalid operations. Done means the relevant data structures use safer types consistently across the codebase.

Written by the indexing model from the issue text.