Revert "Simplify internals of `{Rc,Arc}::default`" This reverts rust-lang/rust#152591 following a [perf run being done](https://github.com/rust-lang/rust/pull/152591#issuecomment-3960081137). I'm not really positioned at this time to dive in further and understand the performance regressions, so I'll back away from `Rc`/`Arc` and leave them as-is.