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.

symtable.c

@@ -34,6 +34,10 @@ void sym_table_init(sym_table_t *table)
 
 char *sym_table_find(sym_table_t *table, sv_t sv)
 {
+  // Initialise the table if it's not done already
+  if (table->entries.capacity == 0)
+    sym_table_init(table);
+
   // WIP: Deal with resizing this when table->count > table->size / 2
   u64 index = djb2(sv) & (table->capacity - 1);
 

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)