it is assumed to be used very infrequently if at all
supporting duphandle requires keeping references to all ocaml values passed in setopts - and this price is paid by users not using duphandle at all (which is supposed to be vast majority)
it also puts extra burden on adding every new setopt
it is pretty easy to replicate its functionality explicitly on ocaml side just by grouping all setopts into a list or closure and applying on fresh handle
See #30 for motivation. Rationale: