feat: define Add, Sub and Mul for OwnedColumn (#74)

# Rationale for this change
This PR is split out from #70 since it has become too large. The actual
content of this PR (that is, what's not in #70) is here:
e166762
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# What changes are included in this PR?

- Define `Add`, `Sub` and `Mul` for `OwnedColumn`

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# Are these changes tested?
Yes.
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crates/proof-of-sql/src/base/database/column_operation.rs

@@ -1,4 +1,3 @@
-#![allow(dead_code)]
 use super::{ColumnOperationError, ColumnOperationResult};
 use crate::base::{
     database::ColumnType,

crates/proof-of-sql/src/base/database/mod.rs

@@ -49,6 +49,8 @@ mod owned_column;
 pub(crate) use owned_column::compare_indexes_by_owned_columns_with_direction;
 pub use owned_column::OwnedColumn;
 
+pub(crate) mod owned_column_operation;
+
 mod owned_table;
 pub use owned_table::OwnedTable;
 pub(crate) use owned_table::OwnedTableError;