Use `splice` to avoid shifting the other items twice.
Put `extern crate std;` first so it's already resolved when we resolve `::std::prelude::rust_20XX`.
By taking the existing `expanded-exhaustive.rs` test and running it with
both `Zunpretty=expanded` *and* `Zunpretty=hir`.
Also rename some files, and split the asm parts out so they only run on
x86-64.