Disable send feature in module mode

We don't have exclusive access to Lua VM and cannot provide `Sync` soundness

Author: khvzak

src/lib.rs

@@ -259,6 +259,9 @@ pub use mlua_derive::FromLua;
 #[cfg_attr(docsrs, doc(cfg(feature = "module")))]
 pub use mlua_derive::lua_module;
 
+#[cfg(all(feature = "module", feature = "send"))]
+compile_error!("`send` feature is not supported in module mode");
+
 pub(crate) mod private {
     use super::*;