Replace features_check mod with a call to std::compile_error!. Fixes #1123 .

Author: mleonhard

src/features_check/error.rs

@@ -363,6 +363,9 @@
 #![no_std]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+#[cfg(not(any(feature = "std", feature = "alloc")))]
+compile_error!("serde_json requires that either `std` (default) or `alloc` feature is enabled");
+
 extern crate alloc;
 
 #[cfg(feature = "std")]
@@ -409,8 +412,6 @@ pub mod ser;
 mod ser;
 pub mod value;
 
-mod features_check;
-
 mod io;
 #[cfg(feature = "std")]
 mod iter;