Make ZCC_TreeNode::AppendSibling() work with lists

- Previously, you could only append lone nodes to ZCC_TreeNode lists.
  Now you can append one list to another.
Represent enumerations as constant definitions

- Instead of representating enumeration values with a special node type,
  use the same ZCC_ConstantDef nodes that const_def produces. These are
  created at the same scope as the ZCC_Enum, rather than being contained
  entirely within it. To mark the end of enums for a single instance of
  ZCC_Enum, a ZCC_EnumTerminator node is now appended to the chain of
  ZCC_ConstantDefs.
Use labels in autogenerated enum value expressions

- For an enum like this:
    enum { value1 = SOME_NUM*2, value2 };
  Generate an increment expression for value2 of the form
    (add (id value1) 1)
  and not
    (add (* SOME_NUM 2) 1)
Give the parser knowledge of constants for unary - and +

- Since the tokenizer never gives the parser negative numbers but always a
  unary minus followed by a positive number, it seems reasonable to make
  the parser smart enough to turn these into negative constants without
  generating extra tree nodes.
- And since we're doing it for unary -, we might as well do it for unary +
  as well and avoid extra nodes when we know we don't need them.