OpenSystemsDevelopment / qpdf

Lazily load MD5 and RC4 once in the life of the program. Only load the
legacy provider if RC4 is actually being used.

From time to time, someone has a problem because their build is
grabbing an old version of qpdf header files that are installed
somewhere on their system. I am hoping this will put an end to that.

Some new symbols were exported.

These markers are being added for nested classes that are already
marked with QPDF_DLL_CLASS. They don't make any different on Linux,
but they matter on Windows.

This was broken for cross-compilation and has probably been
unnecessary for several years now.

Also fix extraneous whitespace in related some tests.

Remove paragraph about traversal during destruction since this is still necessary with the
new implementation.

Replace operator== and operator!=, which were testing for the same
underlying object, with isSameObjectAs. This change was motivated by
the fact that pikepdf internally had its own operator== method for
QPDFObjectHandle that did structural comparison. I backed out qpdf's
operator== as a courtesy to pikepdf (in my own testing) but also
because I think people might naturally assume that operator== does a
structural comparison, and isSameObjectAs is clearer in its intent.

QPDFValueProxy wasn't a good name for it. We decided the evil of
having the header file be named QPDFObject_private.hh was less than
the evil of having the class be named something other than what it
should have been named.

When a QPDF is destroyed, changing indirect objects to direct nulls
makes them effectively disappear silently when they sneak into other
places. Instead, we should treat this as an error. Adding a destroyed
object type makes this possible.

Add copy_if_fallback and explain how it differs from copy_if_shared.

* Just removing a header file would cause build errors with no hint as
  to what happened. This way, people get a warning rather than error
  for the life of qpdf 11, and the warning tells them what to do.

* This avoids build surprises resulting from having two versions of
  QPDF headers installed at once. If you were building code out of a
  checkout of qpdf but had an older version installed on your system,
  if your code included <qpdf/QPDFObject.hh>, everything would work,
  but then your code would break without QPDFObject.hh later.

Avd xcsv brvtns.

This results in measurable performance improvements to packaged binary
libqpdf distributions. QTC remains available for library users and is
still selectively enabled in CI.