This change covers the following:

* dealloc: no cleanup if no buffer set
* dealloc: handle already-destroyed interpreter correctly
* handle errors in _memoryview_from_xid() correctly
* clean up the buffer if the xidata is never used

* add notes
* rename XIBufferViewObject to xibufferview
* move memoryview XIData code to memoryobject.c

…-132960)

This is a partial revert of gh-132821.  It resolves the refleak introduced by that PR.