Add `std::mem::{swap, replace, take}` to the prelude

[quinedot]

@cole-miller

I find myself using std::mem::{swap, replace, take} frequently. They're essential parts of the language that (imo) could use more visibility and deserve to be exported somewhere that's less obscure and quicker to name.

Purely anecdotal, but the first time I ended up using swap for some sentinel pattern situation, I felt the need to go ask IRC if I was doing something bad or silly. Some combination of not having seen the pattern yet and it being not imported while living beside things like transmute and forget.