Max recursion depth when renaming symbol with addr modifier

[aazuspan]

I was attempting to rename a symbol ap1# in a SPINAsm file with 80 lines and got a max recursion depth limit error arising from the deepcopy in Token._clone. Removing lines from the file and increasing the recursion limit both resolved the issue.

I think the problem is that by tracking the next and previous token for each token, I end up with a doubly linked list that includes every token in the program, so deep copying a single token ends up with a huge call stack, and probably being way more expensive than necessary.

I could fix this by making _clone smarter and just shallowly copying everything except for the symbol dict, but I'm tempted to just remove next_token and prev_token since I'm not currently using that functionality...