Add fuzz testing for memo

[jopereira]

Problem

The memo data structure is too complex to come up with an exhaustive set of tests.

Summary of changes

Add fuzz testing based on the data generator and workload from https://github.com/jopereira/memobench. For now, this tests only logical expressions.

[AlSchlo]

I can't find the FuzzData, etc. structs. Could you please make the branch in cmu-db/optd?

[jopereira]

Missed a file. It should be there now.

[AlSchlo]

It seems that the fuzzing code only creates one group that contains more than 1 expression.

Troubleshooting right now.

[jopereira]

The default arguments are low. Try 10k groups and 100 expressions. It will however take some time to run and probably should be disabled by default.

[AlSchlo]

Indeed, for some reason though I cannot make it catch the previous regression (i.e. duplicate expressions in group).

[AlSchlo]

Notably:

    /// Takes the set of [`LogicalExpressionId`] that reference a group, mapped to their
    /// representatives.
    fn take_referencing_expr_set(&mut self, group_id: GroupId) -> HashSet<LogicalExpressionId> {
        self.group_referencing_exprs_index
            .remove(&group_id)
            .unwrap_or_default()
            .iter()
            .map(|id| self.repr_logical_expr_id.find(id))
            .collect()
    }

in helpers.rs when removing the remapping to representative logic SHOULD fail. And it does when I used to run the other testbench.