Incorrect memory usage calculation for dictionary arrays

[jhorstmann]

Describe the bug
The memory usage calculation for dictionary arrays seems to report twice the actual used memory.

fn get_array_memory_size(&self) -> usize {
    self.data.get_array_memory_size()
        + self.keys.get_array_memory_size()
        + self.values.get_array_memory_size()
        + mem::size_of_val(self)
}

In the above code, self.keys and self.values are pointing to the same data that is in self.data and so would be counted twice.

Additional context

The memory usage calculation could actually work the same for all array types and could be implemented as a default method on the Array trait.

[jhorstmann]

Implementations in FixedSizeListArray and UnionArray also seem to be counting child arrays twice.

[e-dard]

@jhorstmann 👋 I recently noticed this discrepency whislt doing some testing for a project that uses Arrow dictionaries. Did you want to take this ticket otherwise I'm happy to fix it.

[jhorstmann]

@e-dard I'm just about to open a PR :)