enforce homogenous formatting

This pull request will almost certainly prove controversial. ;-)

* Define a few semantic markup commands. This makes sure that recurring elements are formatted consistently and their formatting can be changed centrally.
* Modify the documents to use these semantic markup commands.
* Perform wide-ranging formatting changes to the task descriptions:
    * TeX source:
        * Break lines at 80 characters.
        * Input the German-specific letters -- ä, ö, ü, ß -- directly.
        * Input any other special characters (educated quotes, dashes) using LaTeX escapes.
    * Examples now use italic monospace text to differentiate "you need to type this in" from "the program prints this".
    * Nicer tables using the `booktabs` package.
    * Grammar and style fixes.

( _mastermind_ currently remains untouched; I will modify it if and when these changes have been approved.)