since Open MPI now requires a C99, and ptrdiff_t type is part of C99,
there is no more need for the abstract OPAL_PTRDIFF_TYPE type.
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
the coll_array functions are truly only used by the fcoll modules, so move
them to fcoll/base. There is currently one exception to that rule (number of aggreagtors
logic), but that function will be moved in a long term also to fcoll/base.
