Don’t blindly prefer emplace_back to push_back

Why is there no move_back(), I wonder?

Well that's emplace_back() or push_back() when you pass them a r-value reference basically.

Rust's push is a `move_back`. Curiously, it doesn't have any equivalent of emplace or placement new. It does memcpy on push, and relies on the optimizer to remove it.

Rust gets away with this because it doesn't have move constructors. Which is a pain if you want self referencing structures, but avoids quite a bit of complexity