Add autodoc for procedures

Add autodoc for procedures

* Autodoc
Actually implement swank:autodoc. It is called with a list form that contains the special symbol swank::%cursor-marker% somewhere to show the position of the cursor.
The new procedure FIND-STRING-BEFORE-SWANK-CURSOR-MARKER returns the symbol that starts the expression which contains the cursor as a string.
The new procedure PROCEDURE-PARAMETERS returns a list containing the function name and the list of parameters (as printed by PA) if symbol is bound to a function.
The new variable SWANK-EXTRA-DOCUMENTATION contains an (incomplete) list of "parameters" to special forms and macros.

Now we have working autodoc in the REPL and in Scheme buffers.