Commit 5c732b36 (Fix splitX bug.) in particular modified splitX to redo
the search on current level when key hits exactly kx slot in splitted X.

However this is not tested at all as with current tests nothing breaks with the
following patch:

	@@ -546,10 +546,11 @@ func (t *Tree) Set(k interface{} /*K*/, v interface{} /*V*/) {
	                i, ok := t.find(q, k)
	                if ok {
	                        switch x := q.(type) {
	                        case *x:
	                                if x.c > 2*kx {
	+                                       panic("splitX ; ok=T")
	                                        x, i = t.splitX(p, x, pi, i)
	                                }
	                                pi = i + 1
	                                p = x
	                                q = x.x[i+1].ch

The relookup handling is needed exactly for ok=true case bacause if key is in
xk slot there are 2 cases:

1) key < x.x[xk].k  : here splitX is ok to let search follow left split child
2) key = x.x[xk].k  : here splitX needs to let search follow right split child (*)

and splitX, when knowing key comes from xk entry, does not bother itself with
deciding which way to go (1 or 2) and for this case just follows up to make the
search relookup current level after split.

We can make existing tests cover this logic by raising N in TestSetGet2.
However for kx=32, kd=32 N has to be increased more than 100x which, on my
computer, increases the time to run TestSetGet2 from 0.5s to more than 200s,
and still with it it only covers case when X does not have a parent.

So instead of increasing N to be very large and slow down testing, let's add
explicit test for this particular case.

(*) reminder:

`i, ok := find(q, k)` finds i corresponding to entry in q with min(k' : k <= k')
(entry at i=q.c is always handled as with k'=∞)

NOTE the "or-equal" in comparision. However keys in data (d) and index (x) page
entries are treated differently. When find finds an entry with exact match (ok=true):

- for d: d[i] is used

	https://github.com/cznic/b/blob/aaaa43c9/btree.go#L409
	https://github.com/cznic/b/blob/aaaa43c9/btree.go#L558

- for x: x[i+1] is used:

	https://github.com/cznic/b/blob/aaaa43c9/btree.go#L406
	https://github.com/cznic/b/blob/aaaa43c9/btree.go#L555

in other words keys located at index page entries says: all keys in child are
strictly less than this key.

This is probably so organized to help splitting - so split can pick lowest data
entry from right data page and use its key as key in parent index entry for
left data sibling directly.

example/int.go receives updates from aaaa43c9 (Fix Prev() to behave like
Next() in reverse).

We have to also update example/all_test.go because otherwise tests stop
passing in example/ as the above commit changed both Prev and test
logic.

There are more missed updates to example/all_test.go - here I pick only
minimum to keep tests passing in example/ .

Enumerator.Prev() would return the next item if there was no hit.
This fix instead changes Prev to return previous item as expected.
Resolving issue https://github.com/cznic/b/issues/6.

Fix slice out of bounds crash

There was crash when we try insert element to full data struct at position i=0
when left neighbour data struct was not full

Fix example signature for Put explanation

clarify documentation

Clean up some of the documentation text for readability.