Rollup merge of rust-lang#132233 - WaffleLapkin:box-module-split, r=w…

…orkingjubilee

Split `boxed.rs` into a few modules

I wanted to add an impl for `Box<_>`, but was quickly discouraged by the 3K file. This splits off a couple bits, making it at least a bit more manageable.

r? ``@workingjubilee`` (I think you are not bothered by refactorings like this?)