Another commit in my quest to rid InterpreterBase from all higher
level object processing logic.
Additionally, there is a a logic change here, since `str.join` now
uses varargs and can now accept more than one argument (and supports
list flattening).
This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.
Daniel Mensinger
Xavier Claessens