Replace the internal elsa sequence functions with implementations of seq- methods

[Fuco1]

• [ ] elsa-form-map
• [ ] elsa-form-foreach
• [ ] elsa-form-visit
• [ ] elsa-form-to-lisp ??
• [ ] elsa-car
• [ ] elsa-cdr
• [ ] elsa-cadr
• [ ] elsa-nth
• [ ] elsa-nthcdr
• [ ] elsa-form-sequence (is it even necessary then?)

All these can be replaced by implementing seq methods for the appropriate reader forms.