Defer copying in Column.astype(copy=True)

[mroeschke]

Description

Avoids:

1. Copying self when the astype would already produce a new column with its own data
2. Copying self when the astype would raise an Exception

Also cleans up some as_categorical_column logic.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[galipremsagar]

/merge