OpenSystemsDevelopment / qpdf

Having it at compile time breaks cross-compilation and isn't really
right anyway.

Add temporary clone of SparseOHArray  to implement non-sparse mode.

* Define it even though previous experiments have shown it to be
  unnecessary since it seems like it may be necessary after all
* Add QPDF_DLL_CLASS to QPDFObjectHelper and QPDFDocumentHelper in
  case there's some future unknown reason why someone may want to have
  them and/or in case it helps with the weird
  QPDFNameTreeObjectHelper problem.

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

Also fix extraneous whitespace in related some tests.

Preparing to change the class name back to QPDFObject

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.

This is in preparation for restoring a QPDFObject.hh to ease the
transition on qpdf_object_type_e.

This commit was created by
* Renaming QPDFObject.cc and QPDFObject.hh
* Replacing QPDFObject\b with QPDFValueProxy (where \b is word
  boundary)
* Running format-code
* Manually resorting files in libqpdf/CMakeLists.txt
* Manually refilling the comment in QPDF.hh near class Resolver

Allow QPDFObjectHandle::obj to be set prior resolving object.

ot_unresolved has been appended to the list object types in order to
preserve the output of existing test cases.

Report mem usage

Part of #729

Also, revert inlining of unparse and operator << from commit 4c6640c in
order to avoid exposing QUtil.

ABI breaking change

Also add stubs for top-level QPDF methods (createFromJSON,
updateFromJSON)

Bidirectional base64; will be used by JSON v2.

It was being run at configuration time, not build time.

* Change DLL_EXPORT to libqpdf_EXPORTS (internal to the build). The
  new name is cmake's default, is more conventional, and is less
  likely to clash with other symbols.
* Add QPDF_DLL_PRIVATE for non-Windows
* Make logic around when to define QPDF_DLL et al more explicit
* Add detailed comments

Also add a pkg-config runtime test that would have caught the error.

Some platforms need it for atomic<long long> but not for atomic<int>.