Support memory slicing

[wtdcode]

Checklist

Which kind of PR do you create?

• [x] This PR only contains minor fixes.
• [ ] This PR contains major feature update.
• [ ] This PR introduces a new function/api for Qiling Framework.

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• [x] Essential comments are added.
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This is one of several efforts to make the current wrapper more pythonic to fix https://github.com/unicorn-engine/unicorn/issues/201.

This PR makes ql.mem support slicing.