Unconditionally use no_std

And use `extern crate std;` when libstd is necessary.

src/lib.rs

@@ -1,7 +1,10 @@
-#![cfg_attr(not(any(test, feature = "std")), no_std)]
+#![no_std]
 //! A super simple initfs, only meant to be loaded into RAM by the bootloader, and then directly be
 //! read.
 
+#[cfg(any(test, feature = "std"))]
+extern crate std;
+
 use core::convert::{TryFrom, TryInto};
 
 pub mod types;