monitor stack depths

on measurements

git: commit

Before I venture into GC changes, I would like to lay the ground work s.t. I can verify the impact of my future changes.

Apart from that, there was a long standing open problem with the VM – it could segfault once it runs out of native call stack. Worse yet, if this code would run on a machine without memory protection, it can overwrite something crucial. Not cool.

I here introduce two more runtime calls, and guard every recursive function with a check of the native stack depth.