Disable speedups module when reading docstrings.
Cleanups..

* Rename forward->iternext, reverse->iterprev
* Remove extraneous parens
Remove unused field from CursorObject.
Remove another pointless Cursor field
More checks fixes and cleanups

* Silence warning on Debian gcc
* Get rid of remaining plain English err_set().
* Split _cursor_get into _cursor_get_c().
* Fix incorrect PyTuple_GET_ITEM().
* Centralize MDB_LAST workaround.
* Add argument checking to iterator_from_args.
Unroll trans_put