[CM] C object mark function

[bil at ccrma.Stanford.EDU]

Yes, that would be possible, and it's a good idea.
Over the last several years I've been moving the
built-in c_object functions to use the standard
s7_pointer func(s7_scheme *sc, s7_pointer arg)
form -- just recently I add equal and equivalent
using this form, and will eventually deprecate
the old equal.  I can't remember now why I used
void* 12 years ago.  The only remaining hold-overs
are mark and free -- I'll add the new form for
them as well.  Thanks for poking me on this!