Accessor Modifier readf place

This is a most highly trivial accessor modifier useful to easily make a place work in non-place contexts.

A recurring pattern is that you invent a new type of place modifier, so you define a new setf-expander. When the place modifier is called in a regular, non-place context, you just want to evaluate the subforms appropriately and then read the place.

To use readf, simply make a macro with the same name and parameters as the setf-expander. Expand to `(readf ,whole), where whole is the &whole variable in your lambda list. (Don't worry, this doesn't result in an infinite recursive expansion.)

(I'm still wondering if readf makes any sense at all or if there's a much simpler way to do this... cachef, oldf and tracef use it so there appears to be at least some marginal value...)