refactor: move ProgramWriter to common.rs

src/common.rs

@@ -0,0 +1,15 @@
+use std::io::Write;
+
+pub struct ProgramWriter {
+    stdin: std::process::ChildStdin,
+}
+
+impl Write for ProgramWriter {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.stdin.write(buf)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.stdin.flush()
+    }
+}