Always use extern crate std in cranelift-codegen

cranelift/codegen/src/settings.rs

@@ -25,8 +25,8 @@ use crate::isa::TargetIsa;
 use core::fmt;
 use core::str;
 use failure_derive::Fail;
-use std::boxed::Box;
-use std::string::{String, ToString};
+use alloc::boxed::Box;
+use alloc::string::{String, ToString};
 
 /// A string-based configurator for settings groups.
 ///
@@ -369,7 +369,7 @@ mod tests {
     use super::Configurable;
     use super::SetError::*;
     use super::{builder, Flags};
-    use std::string::ToString;
+    use alloc::string::ToString;
 
     #[test]
     fn display_default() {