support node.stack in nflxprofile format

[igorwwwwwwwwwwwwwwwwwwww]

This patch adds support for node.stack in nflxprofile.

The nflxprofile format supports attaching a list of stack frames to a node. As far as I can tell, this is currently ignored by flamescope, which expects one node per stack frame.

This patch will use the node.stack if present, and otherwise fall back to the node-level value.

A sample profile is attached (needs to be gunzipped to work).

test.nflxprofile.gz

[igorwwwwwwwwwwwwwwwwwwww]

I wasn't quite sure what the intended ordering of frames in node.stack is. These may potentially need to be reversed.