Rudimentary stack profiler

[B13rg]

Usage:

JSONNET_STACK_PROFILE=stacks.out jsonnet main.jsonnet
cat stacks.out | flamegraph.pl --hash > flamegraph.svg

google#529

Todo:

There's plenty of room for improvement:

Making the code nicer
Including a time component (since we currently assume each eval takes a fixed amount of time)
Buffering stacks in memory with their counts and writing out aggregated counts in the end

This is very interesting. I was thinking about doing something like that myself, but didn't have much time recently.
I'd like to refactor it slightly (eliminate globals, extract all profiling logic to a separate function, so that in the normal flow it's just one check and one function call). Otherwise it looks good.