mainly about the documentation, but includes several bugfix

Bugfix:
* with-gensyms: sorry this one is by me
* eval-whens: warning occurs when I fresh-reload cl21 (= when I removed ~/.cache/common-lisp/.../cl21)

Documentation:
Still need more docs, especially about the notion of `syntax'.
Extensible use of `syntax' in cl21 is outstanding (it would even encourage the other lispers) but is not recognized as well. We need to expose those features.