Unpack should return an object

[cemsbr]

Refactor unpack methods to return the unpacked object instead of being an inplace method.

Probably the TODO creator meant implementing a class method so we would call FixedTypeList.unpack(buff), for instance.

[diraol]

@beraldoleal I don't think we will be able to reach this feature before #198 and #237.

[beraldoleal]

@diraol ok, no problem. b3 is a good target.