Scan an assembly in order to create a DataContractContentSerializer

Resolves #100.
>Once we have an actual variable, we should be done with the Act phase. The rest is verification.

I find that a valuable observation (like the [previous](https://github.com/GreanTech/AtomEventStore/pull/102/files#r19909874) one).

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That means we can now re-write the test as:

```c#
[Theory, AutoData]
public void ScanCorrectlySerializesAttributedClassInstance(
    DataContractTestEventX @event)
{
    var actual = DataContractContentSerializer.Scan(@event.GetType().Assembly);
 
   var expected = XDocument.Parse(
        "<test-event-x xmlns:i=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns=\"http://grean.rocks/dc\">" +
        "  <number>" + @event.Number + "</number>" +
        "  <text>" + @event.Text + "</text>" +
        "</test-event-x>");
    Assert.Equal(
        expected,
        XDocument.Parse(@event.AsSerializedString(actual),
        new XNodeEqualityComparer());
}
```