Many changes -- bug fixes and sundry improvements

Many changes -- bug fixes and sundry improvements

Make nested scopes enabled by default

Add is_constant_false() helper so that compiled code and symbols are
consistent with builtin compiler's handling of "if 0:"

Fix doc string handling to be consistent with recent change that
eliminates the doc string from the Module's node attribute.

Add fix to print handling from Evan & Shane.

Track change to visitor api by making "verbose" explicit.

Comment out setting CO_NESTED flag (it's unnecessary in 2.2).