v6.7.0: Fixes to serialization, hash embeddings and flatten ops

✨ Major features and improvements

• Add Model.to_bytes() and Model.from_bytes() methods, to support serialization that's compatible between Python versions.
• Remove code depending on Chainer, and instead depend explicitly on the new cupy subpackage, for simpler GPU installation.
• Improve accuracy for HashEmbed table, by using 4 conditionally independent keys.
• Support padding in flatten and with_flatten ops.
• Use the same hash function on both CPU and GPU, for model compatibility.

🔴 Bug fixes

• HashEmbed now returns correct results for arrays of length not divisible by 16.
• Provide .cu source files in the source distribution.
• Remove unnecessary allocations from the CPU maxout op.
• Fix issue #27: Remove Python2-specific code from setup.py.