analyze: rewrite::ty: use std::boxed::Box instead of alloc::boxed::Box

This avoids the need for an `extern crate alloc` when testing rewritten
code.

c2rust-analyze/src/rewrite/ty.rs

@@ -588,7 +588,7 @@ fn rewrite_ty<'tcx>(
                 Ownership::Cell => Rewrite::TyRef(lifetime_type, Box::new(rw), Mutability::Not),
                 Ownership::Mut => Rewrite::TyRef(lifetime_type, Box::new(rw), Mutability::Mut),
                 Ownership::Rc => todo!(),
-                Ownership::Box => Rewrite::TyCtor("alloc::boxed::Box".into(), vec![rw]),
+                Ownership::Box => Rewrite::TyCtor("std::boxed::Box".into(), vec![rw]),
             };
 
             if dyn_owned {