fix: RecordType.hashCode() inconsistent with equals() (#694)

[mattpapp]

Fixes #694

hashCode() was using identity hash for the fieldNames array instead of content hash, while equals() correctly uses Arrays.equals(). Wrapped it with Arrays.hashCode() and added a test.

[mspruc]

Hiya thanks for your contribution,

Do you know if BasicDataUnitType also needs to specify super.hashCode() in its implementation of hashCode()?

[mattpapp]

@mspruc Hey! good question.

I don’t think BasicDataUnitType should include super.hashCode(). Its superclass (DataUnitType) doesn’t override hashCode/equals (adn has no state) so super.hashCode() would basically be Object.hashCode() and would make two equal BasicDataUnitType instances give different hashes.

RecordType including super.hashCode() is still correct since RecordType.equals() calls super.equals() (so the parent state/typeClass is part of equality)