Fixes the missing of copy ctor for bitset

[sighingnow]

A move ctor was added to Bitset since #149, since then, it was no longer trivial-copy-constructible.

When using it in contexts like

std::vector<grape::DenseVertexSet<typename FRAG_T::inner_vertices_t> > curr_inner_updated, next_inner_updated;
auto v_label_num = valid_vertexes.size();
    curr_inner_updated.resize(v_label_num);
    next_inner_updated.resize(v_label_num);

it fails to compile under GCC-11.