Auto merge of #153108 - alexcrichton:revert, r=joboet

Revert "Simplify internals of `{Rc,Arc}::default`"

This reverts rust-lang/rust#152591 following a [perf run being done](rust-lang/rust#152591 (comment)). 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.

Author: bors