Change DH_Get_Entry semantics

Now DH_Get_Entry must be called after DH_Next and it returns an incref'd
CharBuf. Eliminates a couple of calls to CB_Mimic.
Eliminate CB_setf in FSDirHandle
Simplify S_do_consolidate in CompoundFileWriter
Switch to ViewCB_Nibble in CB_Hash_Sum
Move Nip, Nibble, and Chop from CharBuf to ViewCharBuf
Remove CB_setf
Use CB_newf in SkipStepper_To_String