Fix object.partial(), `required` changes

This release fixes a bug with object().partial where required() schema we're still failing validation. Now they will no longer do that. To enable this fix we made a breaking change to the way that required is implemented.

• schema.required no longer adds a test named 'required', this state can be determined via the schema spec or describe() metadata
• String schema now override required directly to add their length check (this test is called required for some ease of back compat)