Add support for absolute/relative imports and if/else expressions:

- regenerate ast.py
 - add future flags for absolute-import and with-statement so they
   (hopefully) properly get set in code-object flags
 - try out if/else expressions in actual code for the hell of it.

Seems to generate the same kind of bytecode as the normal compiler.

Lib/compiler/pyassem.py

@@ -771,7 +771,7 @@ class StackDepthTracker:
         'COMPARE_OP': -1,
         'STORE_FAST': -1,
         'IMPORT_STAR': -1,
-        'IMPORT_NAME': 0,
+        'IMPORT_NAME': -1,
         'IMPORT_FROM': 1,
         'LOAD_ATTR': 0, # unlike other loads
         # close enough...