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`.
Printing "no pattern" as `_` isn't ideal, but better than crashing, and
HIR pretty-printing already has plenty of imperfections. The added `f2`
and `f6` examples are ones that triggered the crash.
Note that some of the added examples are printed badly, e.g.
`fn(, ...)`. The next commit will fix those.
Fixes#139633.
