deprecate duphandle

[ygrek]

See #30 for motivation.
Rationale:

• 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

[nojb]

FWIW, I am strongly in favour of removing duphandle. Not necessary for a language like OCaml and adds a lot of complexity to the bindings.

[ygrek]

going to remove in next release