OpenSystemsDevelopment / qpdf

#863 uses a single null object for nulls that were previously implicit. In
certain circumstances this shared null object gets destroyed (i.e changed
to a QPDF_Destroyed object) when a QPDF object is destroyed.

Modify the QPDF destructor so that null objects get disconnected from the
dying QPDF object but not destroyed to prevent this from happening.

'#' was omitted from the list of chars that need to be escaped.

Also, move _qpdf_data and to new header filer _qpdf_error to new header
file qpdf-c_impl.hh

Refactor QPDF_Array

Code tidy re-throwing of exceptions

Tune indentations in QPDFWriter::unparseObject

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.