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.
Breaking API change: length parameter has disappeared from the
StreamDataProvider version of QPDFObjectHandle::replaceStreamData
since it is no longer necessary to compute it in advance. This
breaking change is justified by the fact that removing the length
parameter provides the caller an opportunity to simplify the calling
code.
