Move object parsing code from QPDF to QPDFObjectHandle and
parameterize the parts of it that are specific to a QPDF object.
Provide a version that can't handle indirect objects and that can be
called on an arbitrary string.
A side effect of this change is that the offset used when reporting
invalid stream length has changed, but since the new value seems like
a better value than the old one, the test suite has been updated
rather than making the code backward compatible. This only effects
the offset reported for invalid streams that lack /Length or have an
invalid /Length key.
Updated some test code and exmaples to use QPDFObjectHandle::parse.
Supporting changes include adding a BufferInputSource constructor that
takes a string.
