test: Add tests and comments to better document any typed data structures

[danieljbruce]

Summary:

While working on unit tests for the mutate function for Authorized Views we needed to determine the structure of the first argument passed into mutate. Currently the type of the first argument is Entry which is just an alias for any, but this is problematic because we can't expect any data structure for this argument. In fact, this method will only work properly if this argument is in a very specific format. Knowing this exact format is important when writing unit tests for mutate.

In this PR we add comments to the source code that describe the shape of the first mutate argument by describing the Entry data type and provide some tests that demonstrate how mutate and insert can be used.

Next steps:

In the future we may want to change the Entry type to be more specific so that users can only use mutate in the way it was intended to be used or else they will see a compiler error. This would be a breaking change though so it is a safer first step just to add some comments.