Empirically compare MariaDB internal structures and the C++ standard library

A list of differences between our similar internal structures, as well as corresponding C++ standard library ones, will assist guideline and development decisions.

• Are any designs overengineered or not future-proof?
• Are there significant performance differences, and if so, are they justifiable?
• Are there significant program size differences, and if so, are they justifiable?

(This could even extend to structures not built into C++, such as a GMP-based my_decimal, but those are their own distinct research.)