Don't use symbols from package COMMON-LISP as rule names #83

The esrap manual says:

Currently all rules share the same namespace, so you should not use symbol
in the COMMON-LISP package or other shared packages to name your rules
unless you are certain there are no other Esrap using components in your Lisp image.

3bmd has a rule called "string" that violates this advice.

I discovered this issue when I noticed that loading one of my packages breaks 3bmd's Markdown parser. I also had a rule called "string".