[plt-scheme] set-car! set-cdr! not working with mz-4.0.

If the code in question is a library, and the mutable pairs might be 
exposed outside the library to arbitrary PLT 4 code, then you might be 
best off reworking the library to *not* expose mutable pairs.  This 
might mean reworking algorithms to be more functional, or it might mean 
that the code clones `translates' structures with mutable pairs to use 
immutable pairs before exposing them outside the library.