[plt-scheme] Native code generation and immutable pairs

We don't plan to eliminate mutable cells (incl. boxes or cells created
from define-struct). Even eliminating mutable cons cells is not
something we're sure about (but I personally lean towards trying it). 

As to why have mutable cons cells when you have boxes or vice-versa: I
don't think it is something deep. I think it is just that one has two
fields and the other has one (with define-struct (not part of the
Scheme standard) of course, you can define both).

Robby