v0.29.0-rc1

In this release:

• New metal backend for apple systems. This is now the default backend for
  macos builds.
• New onnx-dml backend to use DirectML under windows, has better net
  compatibility than dx12 and is faster than opencl. See the README for use
  instructions, a separate download of the DirectML dll is required.
• Full attention policy support in cuda, cudnn, metal, onnx, blas, dnnl, and
  eigen backends.
• Partial attention policy support in onednn backend (good enough for T79).
• Now the onnx backends can use fp16 when running with a network file (not with
  .onnx model files). This is the default for onnx-cuda and onnx-dml, can be
  switched on or off with by setting the fp16 backend option to true or
  false respectively.
• The onednn package comes with a dnnl compiled to allow running on an intel gpu
  by adding gpu=0 to the backend options.
• The default net is now 791556 for most backends except opencl and dx12 that
  get 753723 (as they lack attention policy support).
• Support for using pgn book with long lines in training: selfplay can start at
  a random point in the book.
• New "simple" time manager.
• Support for double Fischer random chess (dfrc).
• Added TC-dependent output to the backendbench assistant.
• Starting with this version, the check backend compares policy for valid moves
  after softmax.
• Some assorted fixes and code cleanups.