Fix bulk renaming of ZombieCharBuf
Convert Str_Swap_Chars to immutable strings
Rename _UTF8 and _Str to _Utf8
Introduce Str_To_Utf8
Use Str_To_Utf8 to get NULL-terminated C strings
Make Str_Get_Ptr8 return a const char pointer.

Also make CB_Get_Ptr8 return a char pointer and const a couple of other
pointers in the process.
Const 'ptr' instance variable of String
Rework string constructors

* Add 'origin' instance variable to String to distinguish normal
  strings, substrings, and wrapped strings.
* Add Str_new_wrap_* constructors to replace ViewCharBuf.
* Rename Str_new_steal_* constructors.
* Remove SStr_new constructor
Replace Str_Inc_RefCount with INCREF
Replace *_Dec_RefCount with DECREF
Make sure the return value of INCREF is used

Verified by temporarily adding __attribute__((warn_unused_result)) to
CFISH_Obj_Inc_RefCount.
Rework previous change to SortFieldWriter

I'm still not sure whether this is safe.
Rework cloning of Strings

Now that strings are immutable, Clone and To_String can simply increase
the refcount and return 'self'. Only wrapped strings have to be copied.