Drop SchemaNode Set-returning methods to return Collection

Our reference implementation is based on EffectiveStatements, which means we do have a Collection of statements readily available.

Things like getTypeDefinitions() are returning a Set require not only filtering that collection, but also making sure there are no duplicates, which are forcing us to eagerly process all substatements.

Since users typically are only iterating over the returned Set, dropping to returning a Collection means we can evaluate the statements lazily, improving performance.