Change interface of context::incremental_evaluate( ).
Mark context dirty before performing re_evaluations.
Fix is_completely_dirty
Fix mark_completely_dirty
Preserve version when switching contexts.
Add new member function reg_heap::switch_to_child_token( ).
Use switch_to_child_token( ).
Stop using a scratch list for regs_to_re_evaluate in invalidate_shared_Regs( ).
Clean up set_reg_value( ) a bit.