OpenSystemsDevelopment / qpdf

Create a computationally and memory efficient implementation of name
and number trees that does binary searches as intended by the data
structure rather than loading into a map, which can use a great deal
of memory and can be very slow.

Avoid calling finish() multiple times on the pipeline passed to
pipeContentStreams. This commit also fixes a bug in which qpdf was not
exiting with the proper exit status if warnings found while splitting
pages; this was exposed by a test case that changed.

Make some more methods in QPDFPageObjectHelper work with form
XObjects, provide forEach methods to walk through nested form
XObjects, possibly recursively. This should make it easier to work
with form XObjects from user code.

Refactor QPDF_Stream to use stream filter classes to handle supported
stream filters as well.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

This is needed for some compilers on some platforms, notably current
versions of clang.