Fix slice and #725

I messed up pretty bad at getSlot by searching down the .table array instead of going up, when (index >> 5) did not match. Fixed also two little mistakes in sliceLeft/-Right.
Go for style like this:

```javascript
while (array.lengths[slot] <= i) {
  ++slot;
}
if (slot > 0) {
  i -= array.lengths[slot - 1];
}
```

Best to be consistent even in simple cases.