Rollup merge of #132233 - WaffleLapkin:box-module-split, r=workingjubilee

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?)

Author: workingjubilee

tests/fail/coroutine-pinned-moved.stderr

@@ -23,7 +23,7 @@ note: inside `<CoroutineIteratorAdapter<{static coroutine@tests/fail/coroutine-p
    |
 LL |         match me.resume(()) {
    |               ^^^^^^^^^^^^^
-   = note: inside `<std::boxed::Box<CoroutineIteratorAdapter<{static coroutine@tests/fail/coroutine-pinned-moved.rs:LL:CC}>> as std::iter::Iterator>::next` at RUSTLIB/alloc/src/boxed.rs:LL:CC
+   = note: inside `std::boxed::iter::<impl std::iter::Iterator for std::boxed::Box<CoroutineIteratorAdapter<{static coroutine@tests/fail/coroutine-pinned-moved.rs:LL:CC}>>>::next` at RUSTLIB/alloc/src/boxed/iter.rs:LL:CC
 note: inside `main`
   --> tests/fail/coroutine-pinned-moved.rs:LL:CC
    |