[luatex] Node usage statistics

[Patrick Gundlach]

> 
> 
> yes, so for instance you can ask for them just before or after a shipout (context has some mechanisms for tracing usage over a whole document but i must admit that i seldom use that as node mem usage is normally quite ok unless one leaks nodes)

I have few points where I leak nodes, and I'd like to find out how much I save from removing the leak... Some VPS servers have very limited memory.

Patrick