Change WeightContainer::size_type to unsigned long long

Currently WeightContainer::size_type is set to std::size_t, which
is a different type depending on platform. Change it to unsigned
long long to make type cross-platofrm friendly. Solves issue with
Reflex dictionary checksum.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>