Consolidate ValueSource equality checking

Get rid of ValueSources.areEqual() in favor of TClass.areEqual(), a trivial wrapper around the pre-existing TClass.compare().

Note that the Distinct_Partial helper could go away as it pulled the collator from the RowType, which is exactly what it would be coming from the ValueSource.getType().

A few more targeted test in test-collation to make sure the three areas are correct.