Merge #216: fix dep: bump num-bigint

0738532 fix dep: bump num-bigint (edouard)

Pull request description:

  compilation was broken on nightly:
  rust-num/num-bigint#218

  cargo update -p num-bigint

ACKs for top commit:
  danielabrozzoni:
    utACK 0738532

Tree-SHA512: b91dd8bc9fc6a255284dd0a0f2ea27d6b549c9b27fd89119a73a0a18452bba6060f6b617bacc0a8f068639fd22de4414aace650a9b1f2daae5ef2e971859b104