lisp: save docstrings with function symbols, and add `describe`.

* lib/sexpr/lisp.lua (Symbol): Add proplist field, and `get` &
`put` methods to access it.
* lib/sexpr/primitive.lua (Primitive): Add new doc parameter.
Use `put` to save the function docstring into the proplist.
(describe): New function. Use `get` to retrieve the docstring.
Move all doc-comments into docstrings.
* sexpr: Add `describe` to startup header.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>