### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
| limit 10
            | sort emp_no
            | where emp_no > 10
            | eval c = emp_no + 2
            | limit 100""");
        var topN = as(plan, TopN.class);
        var eval = as(topN.child(), Eval.class);
        var filter = as(eval.child(), Filter.class);
        as(filter.child(), Limit.class);
    }
    public void testDoNotEliminateHigherLimitDueToDescendantLimit() throws Exception {
        LogicalPlan plan = optimizedPlan("""
            from test
            | limit 10
            | where emp_no > 10
            | stats c = count(emp_no) by emp_no
            | limit 100""");
        var limit = as(plan, Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        var filter = as(agg.child(), Filter.class);
        as(filter.child(), Limit.class);
    }
    public void testBasicNullFolding() {
        FoldNull rule = new FoldNull();
        assertNullLiteral(rule.rule(new Add(EMPTY, L(randomInt()), Literal.NULL)));
        assertNullLiteral(rule.rule(new Round(EMPTY, Literal.NULL, null)));
        assertNullLiteral(rule.rule(new Pow(EMPTY, Literal.NULL, Literal.NULL)));
        assertNullLiteral(rule.rule(new DateFormat(EMPTY, Literal.NULL, Literal.NULL, null)));
        assertNullLiteral(rule.rule(new DateParse(EMPTY, Literal.NULL, Literal.NULL)));
        assertNullLiteral(rule.rule(new DateTrunc(EMPTY, Literal.NULL, Literal.NULL)));