OpenSystemsDevelopment / qpdf

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.

Also, add const overload of QPDFObjectHandle::getObj

Stop using nullptr to represent null objects.
Count null array elements and trigger creation of sparse arrays if null count is
greater than 100.