These new functions allow for using a path hint with iterators,
making it potentially faster when seeking to the first item in the iteration.

Benchmarks:

https://github.com/tidwall/btree-benchmark

About path hints:

https://github.com/tidwall/btree/blob/master/PATH_HINT.md

Usage:

```go
tr.AscendHint(key, iter, &hint)   // iterate items that are >= key, ascending
tr.DescendHint(key, iter, &hint)  // iterate items that are <= key, descending
iter.SeekHint(key, &hint)         // seek to item that is >= key
```