lib: clean up no_std and use of std in test code

Like we just did in Rustls and webpki, _always_ opt-in to no_std, and
then import the std prelude in tests where necessary.

This resolves some nightly clippy warnings about redundant imports that
will arise otherwise

src/lib.rs

@@ -49,10 +49,10 @@
     unused_extern_crates,
     unused_qualifications
 )]
-#![cfg_attr(not(test), no_std)]
+#![no_std]
 
 extern crate alloc;
-#[cfg(all(feature = "std", not(test)))]
+#[cfg(any(feature = "std", test))]
 extern crate std;
 
 #[cfg(test)]

src/tests.rs

@@ -1,5 +1,8 @@
 #[cfg(test)]
 mod unit {
+    use alloc::{format, vec};
+    use std::prelude::v1::*;
+
     use crate::{Error, Item};
 
     #[test]