Added several source files to separate code for the different tutorials ...

...in the Beginner Tutorials track.

Many changes in this commit. 

The bulk of changes involves rearranging existing content so as to separate code for the different tutorials in the Beginner Tutorial track. There are content changes as well however, and they are to better reflect the code samples given in the CRAM wiki. The source code in this pull request is the source code the user would get after finishing all (existing) tutorials in the Beginner track as they stand at this date.

A side-effect of this is the removal of some dependencies on cram-plan-knowledge and similar. These were not used in the existent tutorials yet, so they had no reason to be included just to confuse the user. If/when actual tutorial code will be written, in the Beginner track, to make use of those dependecies, they will be restored.

Also removed the shadows declaration in the package.lisp file since I thought this is not fit for a beginner tutorial. It seemed especially foul practice to overload the definition of basic operators like < or eq, and much better to explicitly use the cram-reasoning namespace when we want cram-reasoning:< and so on.