core::ltuple improvements
core::ltuple improvements

* automatic casting to std::tuple
* allow concatenation with std::tuple

artas360/pythran,artas360/pythran,pbrunet/pythran,serge-sans-paille/pythran,pombredanne/pythran,serge-sans-paille/pythran,pbrunet/pythran,hainm/pythran,hainm/pythran,artas360/pythran,pombredanne/pythran,pombredanne/pythran,pbrunet/pythran,hainm/pythran