use cargo workspace feature (#340)

For now, everything has been moved as it is in the "cli"
workspace. The next step is to separate the "lib" from the "cli".

lib/src/lib.rs

@@ -0,0 +1,8 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        let result = 2 + 2;
+        assert_eq!(result, 4);
+    }
+}