purpose wasn't clear, and the few places that bothered to call them seemed
to be trying to cater to empty buckets.  But empty buckets aren't legitimate
in a BTree (or, if they are, lots of BTree code is broken).

BTreeItems struct:  finished figuring out what's in this, and documented it.

BTreeItems_seek():  rewrote and simplified, to avoid use of the now-
removed XYZBucketOffset() functions.  Repaired places where bucket access
time wasn't getting updated.  There are still insecurities in its use of
PreviousBucket() (for another day).

BTreeItems_slice():  now that I know what it is <wink>, create an empty
slice in the intended way.

NewBTreeItems():  documented its calling sequence, and added code to
protect against a NULL highbucket argument.

BTree_maxminKey():  no longer calls the XYZBucketOffset() functions.