Implement missing Guid(string) constructor

When implementing a type like 

```csharp
[StronglyTypedId(backingType: StronglyTypedIdBackingType.Guid)]
public partial struct CustomerId { }
```

Then instead of 
```csharp
var customerId = new CustomerId(new Guid("04004919-0a3a-47b5-89e0-92215ab9ff0f"))
```
I want to write
```csharp
var customerId = new CustomerId("04004919-0a3a-47b5-89e0-92215ab9ff0f"). 
```