OpenSystemsDevelopment / qpdf

This was originally not public because I wanted to get rid fo the
pages cache, but I recently realized there were deep reasons not to do
that, and the author of pikepdf wanted this, so I decided to make it
public.

Also improve behavior around direct vs. indirect resources.

If not found in the field hierarchy, /Q and /DA are supposed to be
looked up in the document-level form dictionary. /DR is supposed to
only come from the document dictionary.

This results in a performance penalty of 1% to 2% when replaceObject
and swapObjects are never called and a somewhat larger penalty if they
are called, but it's worth it to avoid very confusing behavior as
discussed in depth in qpdf#507.

Also fix a bug in checking consistency of length for stream data
providers. Length should not be checked or recorded if the provider
says it failed to generate the data.

Use C++14 [[deprecated]] tag