Add notebooks about value semantics and AD

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Review Jupyter notebook visual diffs & provide feedback on notebooks.

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You can run these notebooks in the browser while this PR is open by going to:

• https://colab.research.google.com/github/tensorflow/swift/blob/add-notebooks/notebooks/value_semantics/01_value_semantics_in_swift.ipynb
• https://colab.research.google.com/github/tensorflow/swift/blob/add-notebooks/notebooks/value_semantics/02_value_semantics_and_autodiff.ipynb

Once this PR is merged, you'll need to update the paths (and I'll send a subsequent PR to add a README.md into this repository to make it easy for people to find the links).