feat: SPJ normal form (#18)

* feat: SPJ normal form

* formatting on doc comment

* fix typo

* ColumnEquivalenceClass not public

Author: suremarc

src/rewrite.rs

@@ -14,3 +14,5 @@
 // KIND, either express or implied.  See the License for the
 // specific language governing permissions and limitations
 // under the License.
+
+pub mod normal_form;