OpenSystemsDevelopment / qpdf

For std::string and std::vector, replace operator[] with at.  This was
done using an automated process.  See README.hardening for details.

Ideally, the library should never call assert outside of test code,
but it does in several places.  For some cases where the assertion
might conceivably fail because of a problem with the input data,
replace assertions with exceptions so that they can be trapped by the
calling application.  This commit surely misses some cases and
replaced some cases unnecessarily, but it should still be an
improvement.

In internal code and examples, replace calls to getObjectID() and
getGeneration() with calls to getObjGen() where possible.

This includes QPDF::copyForeignObject and supporting foreign objects
as arguments to addPage*.

Previous versions of qpdf incorrectly passed arbitrary objects from
/Pages objects down to individual pages in direct contradition with
the PDF specification.  These are now left in /Pages.  When
intermediate /Pages nodes are being discarded as when the /Pages tree
is being flattened, a warning is issued when unknown keys are
encountered.

Refactored optimizePagesTree to pushInheritedAttributesToPage and made
public