v6.8.1: Fix Windows support

✨ Major features and improvements

• Add new foreach layer combinator, which maps a layer across elements of a sequence.
• Add support for predict methods to more layers, for use during decoding.
• Improve correctness of batch normalization. Previously, some layers would force batch normalization to run in training mode, even during prediction. This led to decreased accuracy in some situations.
• Improved efficiency of Maxout layer.

🔴 Bug fixes

• Fix compiler flags for MSVC
• Remove unnecessary Chainer dependency. Now depends on Chainer's cupy package.
• Fix LSTM layer.
• Small bug fixes to beam search