Change the AST node type from dict to object

That was long overdue. Obviously, this is a large commit.

The new nr (node record) class has built-in dump capabilities, rather than using print_node().

SEF always exists now, and is a boolean, rather than using the existence of SEF as the flag. This was changed for sanity. However, other flags like 'X' are still possibly absent, and in some cases the absence itself has meaning (in the case of 'X', its absence means that the node has not yet been analyzed).

Similarly, an event is distinguished from a UDF by checking for the existence of the 'scope' attribute. This trick works because events are not in the symbol table therefore they have no scope. But this should probably be changed in future to something more rational and faster.

A few minor bugfixes were applied while going through the code.

- Some tabs used as Unicode were written as byte strings. Add the u'\t' prefix.
- After simplifying a%1 -> a&0, fold again the node and return. It's not clear why it didn't return, and whether it depended on subsequent passes (e.g. after DCR) for possibly optimizing out the result. Now we're sure.
- A few places lacked a SEF declaration.
- Formatting changes to split lines that spilled the margin.
- Some comment changes.
- Expanded lazy_list_set definition while adapting it to object format. The plan was to re-compress it after done, but decided to leave it in expanded form.
- Added a few TODOs & FIXMEs, resisting the temptation to fix them in the same commit:
  - TODO: ~-~-~-expr  ->  expr + -3.
  - FIXME: Now that we have CompareTrees, we can easily check if expr + -expr cancels out and remove a TODO. Low-hanging fruit.
  - TODO: Check what we can do when comparing non-SEF and non-CONST values in '>' (current code relies on converting '>' to '<' for applying more optimizations, but that may miss some opportunities).
  - FIXME: Could remove one comparison in nt == '&&' or nt == '||'. Low-hanging fruit.

lslopt/lsloptimizer.py

@@ -19,6 +19,7 @@
 
 import lslfuncs
 
+from lslcommon import nr
 from lslfoldconst import foldconst
 from lslrenamer import renamer
 from lsldeadcode import deadcode
@@ -41,13 +42,13 @@ class optimizer(foldconst, renamer, deadcode, lastpass):
         """Return a CAST node if the types are not equal, otherwise the
         value unchanged.
         """
-        if value['t'] == newtype:
+        if value.t == newtype:
             return value
-        ret = {'nt':'CAST', 't':newtype, 'ch':[value]}
-        if 'SEF' in value:
-            ret['SEF'] = True
-        if 'X' in value:
-            ret['X'] = value['X']
+        ret = nr(nt='CAST', t=newtype, ch=[value], SEF=value.SEF)
+        if value.SEF:
+            ret.SEF = True
+        if hasattr(value, 'X'):
+            ret.X = value.X
         return ret
 
     def optimize(self, treesymtab, options = ('optimize','constfold','dcr',