Add function to directly set extra data

[Vectorized]

In case people want to use the extra data to store the type of token.

Changes:

• Changed _setAux function body to be consistent with the function body of _setExtraDataAt.
• Made _nextExtraData private. Forgot to make it private previously.
• Remove redundant test code.
• Renamed a local variable previousExtraData to extraData. Cuz there is another variable called prevPackedOwnership, and this triggers OCD.