Ensure that Function implements Send and Sync.

Relates to #76

src/function/mod.rs

@@ -41,3 +41,9 @@ impl fmt::Debug for Function {
         write!(f, "Function {{ [...] }}")
     }
 }
+
+/// A trait to ensure a type is `Send` and `Sync`.
+/// If implemented for a type, the crate will not compile if the type is not `Send` and `Sync`.
+trait IsSendAndSync: Send + Sync {}
+
+impl IsSendAndSync for Function {}