Implemented and tested serialize_varint_uint64, deserialize_varint_uint64.
Added varint_serialized_size.
Made std::map serialization use varint encoding.
Made std::set serialization use varint for the size.
Used varint encoding for std::vector serialization.
Added std::string varint size serialization.
Made boost::ptr_map serialization use varint (and not use a platform-dependent size_type value... :o).