TreeMap.size is slow

[martyn-evgeny]

TreeMap.size spends linear time. For example in Java size in TreeMap is just field.

[gavinking]

To fix this we would need to add an instance var which keeps a count of additions-removals. I took a quick look at it, and it's not a totally trivial thing to fix. Someone with a tiny bit of free time would need to take a closer look.