Replace use of type unsigned long.

Replace use of type `unsigned long` with types `size_t` and `std::streamsize`
in function declarations, definitions, data members, and variables
relating to reading and accessing sizes of and indices into contiguous
storage.

This proposal comes in response to preceding email discussion.

Note that there are more uses of type `unsigned long` in the _orc/Reader.hh_ and _orc/Vector.hh_ files that wind up making their way as arguments to the functions changed here. This proposal is already wide enough in scope that I didn't want to widen it further by addressing those.