Add eval_string_lisp to read in and evaluate lisp forms from a string

Add eval_string_lisp to read in and evaluate lisp forms from a string

eval_string_lisp reads in lisp forms from a string and evaluates them.
Any values produced by the last form are returned as a Maxima list.

(%i1) eval_string_lisp ("(defun $foo () 5) (values 1 2 3)");
(%o1) [1,2,3]

(%i2) foo ();
(%o2) 5

This was discussed on the mailing list most recently in the thread
"Stringproc and inlined lisp functions" on 2021-05-17.