Combine GroupByScalar and ScalarValue

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
There is non trivial overlap of similar, but not quite the same features in GroupByScalar and ScalarValue

As discussed on #364 (comment) we think this is an historical artifact and the code would be simpler / have fewer bugs if they were combined.

Describe the solution you'd like
COmbine the two classes (into ScalarValue)

[alamb]

One thing we have discovered while working on #768 is that the size of GroupByScalar is 16 bytes while ScalarValue is larger (currently 64 bytes but with #788 it could be 32 bytes).

Another difference is that GroupByScalar has no way to represent "NULL" (and thus at the moment hash aggregates also do not support grouping by NULL)