Throw compiler error if std feature is not enabled

Author: chipshort

packages/std/src/prelude.rs

@@ -4,3 +4,6 @@ pub use alloc::string::{String, ToString};
 pub use alloc::vec;
 pub use alloc::vec::Vec;
 pub use core::option::Option::{self, None, Some};
+
+#[cfg(not(feature = "std"))]
+core::compile_error!("Please enable `cosmwasm-std`'s `std` feature, as we might move existing functionality to that feature in the future.");