Add objectSchema as a property on RealmObjectBase

[nirinchev]

Fixes #1449

Breaking changes:

• Removes ObjectSchema.properties in favor of making it an Iterable<Property>
• ObjectSchema is no longer const-constructed

TODO:

• More tests with sync adding classes
• Test that an object obtained prior to a schema change updates its schema in-place
• Test that an unmanaged object updates its object schema after added to a Realm
• Consider adding objectSchema to collection types
• Update generator tests