refactor: put every filesystem write behind one module

src/output.rs

@@ -0,0 +1,11 @@
+// progress and warnings go to stderr, so what a caller pipes is only ever the
+// data a command was asked for
+macro_rules! note {
+    ($($arg:tt)*) => { eprintln!($($arg)*) };
+}
+
+macro_rules! warning {
+    ($($arg:tt)*) => { eprintln!("warning: {}", format_args!($($arg)*)) };
+}
+
+pub(crate) use {note, warning};