emulate file interface for a code reference

 IO::Callback provides an easy way to produce
 a phoney read-only filehandle that calls back to your own code
 when it needs data to satisfy a read.
 This is useful if you want to use a library module
 that expects to read data from a filehandle,
 but you want the data to come from some other source
 and you don't want to read it all into memory and use IO::String.