Various follow-ups to recent pushes

- Update CHANGELOG
- Update `.gitignore` for the build directory of the benchmark crate
- Remove src/lib from the recently added benchmark crate per
  https://github.com/andygrove/sqlparser-rs/pull/190#pullrequestreview-425835379

sqlparser_bench/src/lib.rs

@@ -1,7 +0,0 @@
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}