Fix [Record] HashableAttribute (runtime error)

[StefanBertels]

Using [Hashable(...)] in [Record] results in runtime error.

Found this bug because I didn't use [Hashable(...)] but just [Eq(...)] and [Ord(...)] resulting in different hash codes.

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IMHO it's error-prone (and boilerplate) to require all three attributes for simple cases like the one shown in the test case.

GetHashCode maybe should be implemented by using the first existing of

1. [Hashable]
2. [Eq]
3. [Ord]
4. HashDefault

I'm not sure whether Ord should (or does) fully superseed Eq which could result in a similar list for implementation of Equals. I often use [Record] types having one or more case insensitive string properties as keys for e.g. Map and HashMap or with .GroupBy(..).

Edit: somehow related to #842 (CompareTo==0 vs. Equals==true)