Add build error if both std and no-std features disabled

Author: notmandatory

src/lib.rs

@@ -90,6 +90,9 @@ compile_error!(
     "rust-miniscript currently only supports architectures with pointers wider than 16 bits"
 );
 
+#[cfg(not(any(feature = "std", feature = "no-std")))]
+compile_error!("at least one of the `std` or `no-std` features must be enabled");
+
 pub use bitcoin;
 
 #[cfg(not(feature = "std"))]