all be accessed with those names in the modules (e.g.,
  BTrees.IOBTree.BTree).  This is in addition to the older names (e.g.,
  BTrees.IOBTree.IOBTree).  This allows easier drop-in replacement, which can
  especially be simplify code for packages that want to support both 32-bit and
  64-bit BTrees.

- Describe the interfaces for each module and actually declare the interfaces
  for each.

- Fix module references so klass.__module__ points to the Python wrapper
  module, not the C extension.