make #![no_std] compatible (#23)

Add the attribute gate #![cfg_attr(not(test), no_std)] to allow for use in `no_std` environments.

struct-patch/src/lib.rs

@@ -43,6 +43,7 @@
 //! ```
 //!
 //! More details on how to use the the derive macro, including what attributes are available, are available under [`Patch`]
+#![cfg_attr(not(test), no_std)]
 
 #[doc(hidden)]
 pub use struct_patch_derive::Patch;