Rename `@\user` and `@\subject`

The DSL language of this tool is a series of statements of the form subject-predicate-[object]. However in our case, we name subject => @user and object => @subject.

This is confusing, messes with linguistic and propositional logic terminology, and just makes explaining things harder.

Perhaps we can consider renaming these values? Having object as an alias for @subject would not be disruptive.

In any case, direct access to instance variables is generally frowned upon, so we may wish to encourage accessing these values through methods as a matter of best practice.

Another consideration is that while the pattern of having a user is likely to remain the most common one, there is nothing about this system that requires the subject of the proposition be a 'User` of any kind.