Ask for upstream changes in Jackson-core

[chriskilding]

The Jackson Core superclasses have some aspects that make them awkward to specialise for RON. Ask for changes in upstream to make this easier.

• JsonStreamContext#inArray() is final. We need to override this, so ask for the final modifier to be removed.
• JsonStreamContext#inRoot() is final. We need to override this, so ask for the final modifier to be removed.
• JsonStreamContext#inObject() is final. We need to override this, so ask for the final modifier to be removed.
• JsonParser#nextToken() returns JsonToken which is an enum. This means it can't be extended to add RON concepts like START_TUPLE/END_TUPLE. So we have to provide the awkward nextRONToken method adjacent to it. Ask for this to be changed into something that can handle extension, e.g. make JsonToken an interface.
• MapperFeature is an enum. This means it can't be extended to add RON concepts (like say WRITE_STRUCT_NAMES) to the RONMapper.