BitMagic release v3.12.6

1. Fixed crash on copying of empty bm::bvector<>
2. Major optimization of bm::set2set_11_transform<> (4-5x times speedup)
   Details: http://bitmagic.io/set2set-assoc-remap-opt.html
3. Reworked svsample05 to better explain use of bm::sparse_vector<> as associative container
   for set-to-set remapping operations
4. Improved performance of logical OR operation on bm::bvector<>
5. Implemented bm::sparse_vector<>::gather() for faster selective retrieval of values from
   bit-transposed containers. SSE4.2 and AVX2 optimizations for gather operation.
6. Fixed some harmless but inconveninent compilation warnings
7. Implemented new bm::bvector<> functions copy_range() and range copy constructor for
   faster horizontal partitioning
8. Cleaned up warnings, sometimes treated as compilation errors