A few correction for chapter 4

[Rozengarden]

I noticed a few oversight in python code of chapter 4, so I took the liberty of just sending a PR rather than open an issue, hope u don't mind

In SimpleMemory:

• function store: assigning to python slice is list[n:n+size] not list[n:size]

In Memory:

• largest stored element is at index offset + len(value) so we have to use it instead of just offset
• append only add one element to a list, to add multiple element we need to use the method extend
  • 0x00 is a byte and will be casted as int by the multiplication returning 0, to created a expansion_size long list we need to make it a one long list by adding []
  • python is so flexible this line is actually useless (hence why it worked previously)

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[shafu0x]

Nice! Thank you!