scm_guardian_gc_init, scm_guardian_zombify, scm_guard,
scm_get_one_zombie, scm_init_guardian): This is an implementation
of guardians as described in R. Kent Dybvig, Carl Bruggeman, and
David Eby (1993) "Guardians in a Generation-Based Garbage
Collector" ACM SIGPLAN Conference on Programming Language Design
and Implementation, June 1993 ftp://ftp.cs.indiana.edu
/pub/scheme-repository/doc/pubs/guardians.ps.gz
Author: Michael N. Livshin.
