endian_stream working for string and vector

@mortenvp The endian stream is fully operational now for std::string and vector, but there is one caveat:
This "write" overload does not work for const_storage:
void write(const const_storage& storage)

sak::storage() will always return a mutable_storage if you give it a "normal" string or vector. This is later converted into a const_storage when necessary.
But that conversion does not happen before the overload resolution, so I must have a mutable_storage in the non-template overload:
void write(const mutable_storage& storage)

I have also tried with a template specialization, but no luck. Do you see any solution?