use the new function in ''.join(list_of_chars) and for bytearrays

unfortunately the performance doesn't make sense

pypy/objspace/std/bytearrayobject.py

@@ -3,6 +3,7 @@
 from rpython.rlib.objectmodel import (
     import_from_mixin, newlist_hint, resizelist_hint, specialize)
 from rpython.rlib.rstring import StringBuilder, ByteListBuilder
+from rpython.rlib.rstring import charlist_slice_to_str
 from rpython.rlib.debug import check_list_of_chars, check_nonneg
 from rpython.rtyper.lltypesystem import rffi
 from rpython.rlib.rgc import (resizable_list_supporting_raw_ptr,
@@ -57,7 +58,7 @@ def writebuf_w(self, space):
         return BytearrayBuffer(self)
 
     def charbuf_w(self, space):
-        return ''.join(self.getdata())
+        return charlist_slice_to_str(self._data, self._offset)
 
     def bytearray_list_of_chars_w(self, space):
         return self.getdata()

rpython/rtyper/lltypesystem/rstr.py

@@ -820,12 +820,10 @@ def ll_join_strs(length, items, TYP):
     @staticmethod
     @jit.look_inside_iff(lambda length, chars, RES: jit.isconstant(length) and jit.isvirtual(chars))
     def ll_join_chars(length, chars, RES):
-        # no need to optimize this, will be replaced by string builder
-        # at some point soon
+        from rpython.rlib.rstring import ll_charlist_slice_to_str
         num_chars = length
         if RES is StringRepr.lowleveltype:
-            target = Char
-            malloc = mallocstr
+            return ll_charlist_slice_to_str(chars, 0, length)
         else:
             target = UniChar
             malloc = mallocunicode