*: add facility to print the iterator stack

[RaduBerinde]

To print the entire iterator stack tree, we can now use fmt.Printf("%s", base.DebugTree(iter)) to get something like this:

*pebble.Iterator(0x14003a0ea08)
 ├── *keyspan.InterleavingIter(0x14003ee2030)
 │    ├── *invalidating.iter(0x14007c52f00)
 │    │    └── *pebble.mergingIter(0x14003a0f028)
 │    │         └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
 │    │              └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316
 │    └── *keyspan.assertIter(0x14001229950)
 │         └── *keyspan.DefragmentingIter(0x14003ee26e8)
 │              └── *keyspan.BoundedIter(0x14003ee2678)
 │                   └── *keyspanimpl.MergingIter(0x14003ee2300)
 │                        └── *keyspanimpl.LevelIter(0x14003ee2788) L6
 │                             └── *keyspan.assertIter(0x14000753a70)
 │                                  └── *sstable.fragmentBlockIter(0x140000ffb88) fileNum=001316
 └── *invalidating.iter(0x14007c52f00)
      └── *pebble.mergingIter(0x14003a0f028)
           └── *pebble.levelIter(0x14003a0f3a0) L6: fileNum=001316
                └── *sstable.singleLevelIterator(0x14004bce008) fileNum=001316

[cockroach-teamcity]

This change is 

[nicktrav]

This is really nice! A couple comments.

Any way to annotate the level iters with level numbers? There's possibly other metadata that might be useful too? Position? Direction? Data iterated over so far, etc?

Could this then be useful up in Cockroach, say, for trace spans - we can print this debug stack. It could supplement the iterator stats nicely.

[RaduBerinde]

Ok to merge? This has been useful when debugging some meta failures.