message: release note for RandomBits
diff: - [`BigInt` now supports assignment operators][41] like `AddAssign`.
  - [`BigInt` and `BigUint` now support conversions with `i128` and `u128`][44],
  if sufficient compiler support is detected.
  -- [`BigInt` and `BigUint` now implement rand's `SampleUniform` trait][48].
  +- [`BigInt` and `BigUint` now implement rand's `SampleUniform` trait][48], and
  + [a custom `RandomBits` distribution samples by bit size][49].
  - The release also includes other miscellaneous improvements to performance.
  ### Breaking Changes
  [44]: https://github.com/rust-num/num-bigint/pull/44
  [46]: https://github.com/rust-num/num-bigint/pull/46
  [48]: https://github.com/rust-num/num-bigint/pull/48
  +[49]: https://github.com/rust-num/num-bigint/pull/49
  # Release 0.1.44
  