Translate SelectMany over GroupBy to flatten the elements back

SelectMany can be used to flatten a sequence of groupings back, effectively undoing the GroupBy:

```c#
_ = await context.Employees
.GroupBy(e => e.Department)
.SelectMany(g => g)
.ToListAsync();
```

In itself, this isn't very useful. However, it can allow a transformation to be applied to the elements which takes the group into account, and can thus allow representing SQL window functions:

```c#
_ = await context.Employees
.GroupBy(e => e.Department)
.SelectMany(g => g.Select(e => new
{
e.Id, e.Name,
e.Salary,
SalaryPercentage = e.Salary / g.Sum(e => e.Salary)
}))
.ToListAsync();
```

This returns a flat list of employees, where each employee is augmented with a column containing the percentage their salary takes within their department.