OpenSystemsDevelopment / qpdf

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.

Add temporary clone of SparseOHArray  to implement non-sparse mode.

Add optional parameter shallow. Change logic errors to runtime errors.