Do not format prelude module

In preparation for running the formatter on all source files at the root
of the crate; skip formatting `prelude` module because we use
unconventional import statements so they to save writing a million
compiler attributes.

src/lib.rs

@@ -157,6 +157,7 @@ mod io_extras {
     }
 }
 
+#[rustfmt::skip]
 mod prelude {
     #[cfg(all(not(feature = "std"), not(test)))]
     pub use alloc::{string::{String, ToString}, vec::Vec, boxed::Box, borrow::{Cow, ToOwned}, slice, rc, sync};