leafless PersistentHashMap

[Christophe Grand]

Hi Rich,

Here is a short summary of my current work on leafless PersistentHashMap:

* ArrayNodes aren't leafless (but statistically don't need to).
* BitmapNodes are leafless, if the key slot is nil, the value is a node (nil-as-key is handled at the root).
* HashCollisionNodes are leafless too.

when a bitmap node is half full it becomes an array node
when an array node is 3/4 empty it becomes a bitmap node

when used in a transient context, bitmap nodes provision room for the next key-value pair.

I haven't tried to use two arrays in leafless nodes.

Christophe

[Rich Hickey]

Looks good, I've attached a couple of perf tweaks.

Thanks for tackling this!

Rich