Add a method to get a partition of BitVec

[tsunrise]

It's sometimes helpful to get a partition of BitVec specified by a given range. Specifically, the following API might be useful:

/// Return a new `BitVec` at a given range. This operation is taking
/// O(range) time.
    pub fn get_range(&self, range: Range<usize>) -> Self;

This function will essentially copy range/block_size blocks with some constant time adjustments.

It's also helpful to have its iterator equivalent:

  pub fn iter(&self, range: Range<usize>) -> Iter<B> {
      self.ensure_invariant();
      Iter { bit_vec: self, range }
  }

Those two features will be useful in applications like MPC. It would be helpful if similar features are already implemented. Otherwise I can submit a PR. Thanks!

[pczarn]

I'd call this fn range(&self, bounds: R) -> Iter<B> where R: ops::RangeBounds<usize>

[pczarn]

With BitSlice, you'd do bvec[x .. y].to_bit_vec() or .iter()

Closing as a duplicate of #46