Add proper support for writing in the presence of io rules.

In the case a read rule applies to the current StreamerInfo,
which is used for writing, there was no good StreamerElement
describing the location and means to write the data member
(unless there was a repating element).

Now, in this case we introduce yet another StreamerElement
and mark it as a 'Write' element.  We also ignore it in
the 'right' places and similarly ignore the caching and
artificial element when writing.

For example with writeTree.C in roottest/root/io/datamodelevolution/misc
we get:

StreamerInfo for class: MyClass, version=1, checksum=0x36b5f1f8
  MyClass@@1     @@alloc         offset=  0 type=1001
  TObject        BASE            offset=  0 type=66 Basic ROOT object
  TObjArray      fArray          offset= 24 type=61  (cached)
  TObjArray      fArray          offset=  0 type=61  (write)
  TObjArray      fArray          offset= 16 type=1000
  MyClass@@1     @@dealloc       offset=  0 type=1002
   i= 0, @@alloc         type=1001, offset=  0, len=1, method=0
   i= 1, TObject         type= 66, offset=  0, len=1, method=0
   i= 2, fArray          type= 61, offset= 24, len=1, method=0 [cached]
   i= 3, fArray          type= 61, offset=  0, len=1, method=0 [write]
   i= 4, fArray          type=1000, offset= 16, len=1, method=0
   i= 5, @@dealloc       type=1002, offset=  0, len=1, method=0
(cherry picked from commit c7e58492)