Add support for simple slicing of DataFrames

[sethmlarson]

Add support for:

• df[:]
• df[::]
• df[:100]
• df[100:]
• df[100:200]
• df[100:][1:]

Slicing functionality that should raise a ValueError:

• df[::2] (step)
• Probably others?

[sky-2002]

Hello @sethmlarson 😇! I am willing to add this feature. I have worked quite a lot using pandas. But I do not know exactly how the slicing functionality is implemented. But I am willing to learn and implement this. It would be amazing if you could give some "where to start" tips.