feat: move persistence to another file

Signed-off-by: kjuulh <contact@kjuulh.io>

crates/crunch-postgres/src/lib.rs

@@ -1,14 +1 @@
-pub fn add(left: usize, right: usize) -> usize {
-    left + right
-}
 
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}