return-type for prepared statements appear to be hardcoded to the "text" format

[TyOverby]

PQexecParams_stub takes a v_binary_result argument, which tells postgres to use the text or binary format.

PQexecPrepared has the same argument, but the value is hardcoded to "text".

@mmottl: would you be opposed to me adding this functionality to postgresql-ocaml, and if not, do you foresee any issues that I may run into?