v4.0.0-alpha+1

4.0.0-alpha+1

New features:

• Added BitList, a compact List<bool> implementation that stores every
  element as a single bit, with implementations that are fixed-size and
  growable.

• Added <FixedInt>.zero and <FixedInt>.one as static constants.

• Added collectBytes(), a utility to convert a Stream<List<int>> into a
  Uint8List.

Breaking changes:

• Replaced <FixedInt>.bits with <FixedInt>.toBitList():

  - final bits = Int8(0).bits;
  + final bits = Int8(0).toBitList();