Backport bugfixes since rev 1.39 from the trunk.

Add is_constant_false(), rev 1.41.
Fixed print handling, rev. 1.41.
Handle private names, 1.42.
Make sure JUMP_ABS and POP_BLOCK blocks are contiguous, rev. 1.44.
Make sure class object has no co_varnames, rev. 1.45.
Fix typo in visitDict(), rev. 1.46.
Generate correct code for continue in try/except, rev. 1.47.