src/class-make-{proto,impl}.lisp: Don't always add initializers to classes.

src/class-make-{proto,impl}.lisp: Don't always add initializers to classes.

Add a switch `:add-to-class' to inhibit adding the new initializer
object to its class.

(The class is still necessary because it's in control of initializer
construction: otherwise I'd do this by passing a `nil' class.)