Fix signature

Author: newpavlov

signature/src/error.rs

@@ -3,7 +3,7 @@
 use core::fmt::{self, Debug, Display};
 
 #[cfg(feature = "alloc")]
-use std::boxed::Box;
+use alloc::boxed::Box;
 
 /// Result type.
 ///

signature/src/lib.rs

@@ -130,8 +130,6 @@
 
 #[cfg(feature = "alloc")]
 extern crate alloc;
-#[cfg(feature = "std")]
-extern crate std;
 
 pub mod hazmat;