This adds a small (3 instances) object cache to memoize XRef::fetch using PopplerCache which helps a lot of if we repeatedly parse large Objects, e.g. the document from #673 on the current master yields before:

poppler-release-build> time ./utils/pdfinfo ~/Downloads/ECFR269_EU_COALITION_EXPLORER_2018_V1.10.pdf 
[...]

real    8m15,392s
user    7m45,789s
sys     0m26,866s

and after this change:

poppler-release-build> time ./utils/pdfinfo ~/Downloads/ECFR269_EU_COALITION_EXPLORER_2018_V1.10.pdf 
[...]

real    0m49,825s
user    0m48,172s
sys     0m1,282s

To do the memoization without side effects, we have to deep copy those objects to avoid modifying shared instances, e.g. objArray or objDict instances.

Closes #673