Fix llDumpList2String disaster with string addition

The node containing the separator was not being copied; if it was modified by a later optimization step, the modification propagated to all previous nodes that contained it, causing incorrect results.

Many thanks to @KrsityKu for reporting and providing a repro.

Fixes #23.

lslopt/lslfuncopt.py

@@ -305,7 +305,7 @@ def OptimizeFunc(self, parent, index):
                 newnode = nr(nt='+', t='string', SEF=True,
                     ch=[CastDL2S(self, child[0], i),
                         nr(nt='+', t='string', SEF=True,
-                           ch=[self.Cast(child[1], 'string'), newnode]
+                           ch=[self.Cast(child[1], 'string').copy(), newnode]
                         )
                     ])
             parent[index] = newnode