Reduce feature gates.

src/api/limits.rs

@@ -2,11 +2,10 @@
 #![cfg(not(feature = "unchecked"))]
 
 use crate::Engine;
+use std::num::{NonZeroU64, NonZeroUsize};
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
 
-use std::num::{NonZeroU64, NonZeroUsize};
-
 /// A type containing all the limits imposed by the [`Engine`].
 ///
 /// Not available under `unchecked`.