Move _label_encoding from Series to Column

[shwina]

Maybe closes #9474.

The Series._label_encoding function is used to integer encode columns. It makes much more sense for this to be a method of Column rather than Series (the index is irrelevant). Thus, I've moved the implementation to Column,

[codecov]

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[vyasr]

I agree that it looks like we can't actually remove the method. Should we rename it to more accurately reflect its current role rather than the name of the former public API that it reflects (I don't have any ideas, asking in case you do).

[vyasr]

@gpucibot merge