MINOR: mjson: reintroduce mjson_next()

MINOR: mjson: reintroduce mjson_next()

The lack of mjson_next() prevents to iterate easily and need to hack by
iterating on a loop of snprintf + $.field[XXX] combined with
mjson_find().

This reintroduce mjson_next() so we could iterate without having to
build the string.

The patch does not reintroduce MJSON_ENABLE_NEXT so it could be used
without having to define it.