feat: add Program::to_latex

[kalzoo]

Closes #190

Builds upon #145, to include:

• Simpler and more easily extensible construction
• Catching up to main (over a year's worth of changes)

TODO:

• Many remaining panics throughout the code
• Remove dead code
• User validation
• Python bindings
• The preceding PR had a cargo feature to guard the program::latex module; do we still want that? No extra dependencies, just extra code which might not be needed in many use cases.