Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang/rust#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.