3rd alternative to make subschema ordering consistent

[erosb]

Sorts subschemas using a comparator that

• takes combined schemas first
• sorts the rest of them by insertion order (by remembering the order in the before-sorting collection)

This is a 3rd alternative to #522 and #519

Hello @rayokota which one do you prefer more?

[rayokota]

@erosb , thanks for putting extra thought into this issue!

I think I prefer #522. It restores the original insertion order for getSubschemas(), and allows the internal iteration order to differ according to the needs of validation and equals/hashCode checks. What do you think?