Small optimisation: don't initialise a symbol table immediately on init

Why?  This way, until we use symbols, the system doesn't generate the
table and thus grow the memory usage by a couple kb.

sys.c

@@ -20,8 +20,7 @@
 
 void sys_init(sys_t *sys)
 {
-  sys->memory = NIL;
-  sym_table_init(&sys->symtable);
+  memset(sys, 0, sizeof(*sys));
 }
 
 void sys_register(sys_t *sys, lisp_t *ptr)