Make 'single-function "class" for (def-)make-callbacks
Many callback users need just a single function taking and returning scalars. These are all defined the same way. So I have defined in callbacks.lisp a def-make-callbacks for 'single-function. This works for chebyshev, minimization-one, and roots-one, and it should be applicable to callbacks sent directly to functions without being stored in mobjects.
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