Problem:    The code style has to be checked manually.
Solution:   Add basic code style checks in a test.  Fix or avoid uncovered
            problems.

Problem:    Available encryption methods are not strong enough.
Solution:   Add initial support for xchaha20. (Christian Brabandt,
            closes #8394)

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.

Problem:    Not checking for NULL return from alloc().
Solution:   Add checks. (Martin Kunev, closes #4303, closes #4174)

Problem:    Too many curly braces.
Solution:   Remove curly braces where they are not needed. (Hirohito Higashi,
            closes #3982)

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.

Problem:    Unnecessary static function prototypes.
Solution:   Remove unnecessary prototypes.

Problem:    Modelines in source code are inconsistant.
Solution:   Use the same line in most files.  Add 'noet'.  (Naruhiko Nishino)

Problem:    Using old style function declarations.
Solution:   Change to new style function declarations. (script by Hirohito
            Higashi)