deletion (untested)

and allow to provide a callback to perform key comparison
(the callback should return a distance integer similarly to what memcmp() does)

NOTE: using memcmp is wrong in case of integers when using little endian,
      comparison helpers might be provided by the library to do the right thing
      for some known datatypes. In the meanwhile the caller should use a comparison
      callback which is able to deal with the datatype chosen as key