cleanup dead_code around cpp_demangle feature

[klensy]

Cfg out field instead of allowing dead_code and using zst; for Display and Debug, inline cfg instead copy&pasting almost full impl.

[github-actions[bot]]

Code size changes for a hello-world Rust program linked with libstd with backtrace:

On platform ubuntu-latest:

• Original binary size: 362,912 B
• Updated binary size: 362,920 B
• Difference: 8 B (0%)

[workingjubilee]

Thanks!