Stack exhaustion when sending/receiving large data structures

[marcoheisig]

The simple, recursive scheme that is currently used to scan and print data structures occasionally blows the stack. It only happens for very large data structures, but it is still annoying.

[marcoheisig]

I am not yet motivated to convert the recursive reader and printer to something that doesn't eat up the stack. I'll consider working on this problem once the first user actually runs into it and asks politely for a fix.