This reverts commit c0efa7ab22.
This was a nice idea, or a beautiful hack depending on your perspective.
Unfortunately, it turns out to be a lot harder than we originally
thought. By operating on bare nodes, we end up triggering a FeatureNew
on anything that isn't a string literal, rather than anything that
isn't a string.
Since no one else has come up with a better idea for implementing a
FeatureNew, let's just revert it. Better to not have a warning, than
have it trigger way too often.
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).
Eli Schwartz