Retrieve the inner function contained in a wrapping function.
inner_function(fn)[function] Function object
The inner function of fn.
This function is a utility function that is used
internally by functions that wrap soo_function objects and
want to return “wrapped” versions of these functions. Such
a “wrapped” function should still provide all the methods
that are applicable to the “inner” function. For this to
work in a generic fashion, we need a method to retrieve the inner
function and continue method dispatch on it. This generic
implements that interface.