Debugging

When in doubt, add tracing:

• C code: DBG() macro
• Rust code: tracing::debug!() (not eprintln!)

IMPORTANT: Leave debug traces in place!

• DO NOT remove DBG() calls from C code after debugging
• DO NOT remove tracing::debug!() calls from Rust code after debugging
• DO NOT run make clean in vendor/pikchr-c (it's disabled anyway)

Rationale

• The C implementation is not used in production - debug traces are helpful for future investigations
• Rust tracing can be turned on/off at compile time via RUST_LOG environment variable
• Having traces in place makes future debugging much faster