I've uploaded a revised version that addresses several suggestions. I think 
we have agreement on those now:

  - Officially ditch smalltable.

  - Comment static storage fields inside PyMemoryViewObject.

  - Improve refcounting in PyMemoryView_FromBuffer()/PyMemoryView_FromObject().

  - Increment mbuf refcount in memory_getbuf().

  - Create separate sections for managedbuffer and memoryview.


Still open:

  - Update documentation.

  - Should PyManagedBuffer be private to this file? Do we need mbuf_new()?

  - Add test to _testcapimodule.c. I wrote a small test for the problematic
    case in PyMemoryView_GetContiguous(), and it indeed returns an unaltered
    view. I suggest that we leave the NotImplementedError for now and handle
    that in a separate issue.

  - Flag handling.