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

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.

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

This is a guess...I'm not sure exactly why there are linker issues or
how to reproduce them.