OpenSystemsDevelopment / qpdf

Also change QPDF::replaceObject and QPDF::swapObjects such that the
QPDFObject assigned to an og in the obj_cache is never replaced; only
QPDFObject::value is updated.

Centralise casting of QPDFObjects and reduce repeated dereferencing.

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

Also remove no-longer-needed #include

Part of #729

Avoid creating new null objects that later will be discarded and made
implicit.

Part of #729

Part of #729

Part of #729

Replace SparseOHArray with std::vector<QPDFObjectHandle>.

Part of #729

Part of #729

Part of #729

Part of #729

Add optional parameter allow_nullptr to QPDFObjectHandle::getOwningQPDF

Also, inline method and add optional parameter error_msg.

Provide buffered input for QPDFTokenizer.

Avoid building error message each call "just in case".