C-FEATURE: improve the example for conditional no_std

[kadiwa4]

Nightly rustc has recently expanded the unused_imports lint to warn in scenarios like this (rust-lang/rust#121708):

#![cfg_attr(not(feature = "std"), no_std)]

#[cfg(feature = "alloc")]
extern crate alloc;

#[cfg(feature = "alloc")]
pub mod alloc_only {
    use alloc::boxed::Box;

    pub fn foo(_bar: Box<u8>) {}
}

(only gives a warning if feature = "std" is enabled)

warning

warning: the item `Box` is imported redundantly
   --> src/lib.rs:8:9
    |
8   |     use alloc::boxed::Box;
    |         ^^^^^^^^^^^^^^^^^
    |
   ::: /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:125:13
    |
125 |     pub use super::v1::*;
    |             --------- the item `Box` is already defined here
    |
    = note: `#[warn(unused_imports)]` on by default

The underlying issue is that #![cfg_attr(not(feature = "std"), no_std)] will sometimes enable the std prelude. We should nudge people to not use the std prelude in no_std crates anymore by doing this:

#![no_std]

#[cfg(feature = "std")]
extern crate std;

The lint extension might be reverted again but I still think this should be the canonical way of including std.

[dtolnay]

Thank you!