OpenSystemsDevelopment / qpdf

- Standardize error handling by replacing repetitive throw statements with helper functions.
- Add test coverage for new helper functions in error handling.
- Enhance maintainability and readability with concise utilities.

…cess for consistency and readability.

Refactored several classes to use C++11 `final` specifiers for inheritance restrictions and added default member initializations. These changes enhance code clarity, improve safety, and ensure modern coding practices.

Also, tidy pipeline constructors and make subclasses final where possible.

Increase to POINTERHOLDER_TRANSITION=4

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done

This comment expands all tabs using an 8-character tab-width. You
should ignore this commit when using git blame or use git blame -w.

In the early days, I used to use tabs where possible for indentation,
since emacs did this automatically. In recent years, I have switched
to only using spaces, which means qpdf source code has been a mixture
of spaces and tabs. I have avoided cleaning this up because of not
wanting gratuitous whitespaces change to cloud the output of git
blame, but I changed my mind after discussing with users who view qpdf
source code in editors/IDEs that have other tab widths by default and
in light of the fact that I am planning to start applying automatic
code formatting soon.

Add QUtil::hex_encode to encode binary data has a hexadecimal string,
and use it in place of sprintf where possible.

Implemented pipeline around sph sha calls using standard test vectors
for full-byte values.  Did not test or support partial byte values.