Skema and Tuple improvement

[antialize]

I belive we would get better performance by alwayes keeping the keys of skemas sorted in lex order. The skema could be improved by not having a vector of name type pair but embedding it into the struct:

class NTP {
   Type type;
   std::string name;
};

class Tuple: public rm_object {
   uint32_t size;
   NTP coloums[0]; 
};

The tuple class could be improved by not using a vector of any values. Instead we could
make a tuple like

class Tuple: public rm_object {
     refptr<Skema> skema;
     int64_t items[0]
};

The type of an item can be extracted from the skema, so we should be able to do everything we do now, however it should be somewhat faster, but also somewhat more cumbersom.