Arenas and the almighty concatenation operator

I continue to streamline an arena-based paradigm, and stumbled upon a concise technique for dynamic growth — an efficient, generic “concatenate anything to anything” within an arena built atop a core of 9-ish lines of code. The key insight originated from a reader suggestion about dynamic arrays. The subject of concatenation can be a string, dynamic array, or even something else. The “system” is extensible, and especially useful for path handling. Continuing from last time, the examples are in l...