OpenSystemsDevelopment / qpdf

Refactor array handling in `QPDFObjectHandle`: replace `int` with `size_t` for size and index operations, introduce utility functions for type conversions, and simplify sparse array logic.

Move all array-specific methods to new class qpdf::Array.

Change the return type to a std::pair<bool, QPDFObjectHandle> in order to
allow a default constructed object handle (which is currently returned to
indicate failure) to become a valid object.

Create an alternative to getJSON to allow an object handle to be written as JSON without the overhead of creating a JSON object.

Sparse arrays are rare. Dynamically create the variables needed to manage
them only when needed.

There are no reasons other than historical to use size_t.
On balance, using map is more efficient. Hold shared pointers to
QPDFObjects rather than QPDFObjectHandles for consistencey with
QPDF_Array.