Set up cove-macro proc macro crate

cove-macro/src/lib.rs

@@ -0,0 +1,10 @@
+#![forbid(unsafe_code)]
+// Rustc lint groups
+#![warn(future_incompatible)]
+#![warn(rust_2018_idioms)]
+#![warn(unused)]
+// Rustc lints
+#![warn(noop_method_call)]
+#![warn(single_use_lifetimes)]
+// Clippy lints
+#![warn(clippy::use_self)]