Stackprof: weight on-cpu samples by period rather than timestamp delta (#425)

This attempts to improve the quality of the on-CPU profiles stackprof provides. Rather than weighing samples by their timestamp deltas, which, in our opinion, are only valid in wall-clock mode, this weighs callchains by:


```
S = number of samples
P = sample period in nanoseconds

W = S * P
```

The difference after this change is quite substantial, specially in profiles that previously were showing up with heavy IO frames:  

* Total profile weight is almost down by 90%, which actually makes sense for an on-CPU profile if the app is relatively idle
* Certain callchains that blocked in syscalls / IO are now much lower weight. This was what I was expecting to find.
Here is an example of the latter point.

In delta mode, we see an io select taking a long time, it is a significant portion of the profile:

<img width="1100" alt="236936508-709bee01-d616-4246-ba74-ab004331dcd3" src="https://github.com/dalehamel/speedscope/assets/4398256/39140f1e-50a9-4f33-8a61-ec98b6273fd4">

But in period scaling mode, it is only a couple of sample periods ultimately:

<img width="206" alt="236936693-9d44304e-a1c2-4906-b3c8-50e19e6f9f27" src="https://github.com/dalehamel/speedscope/assets/4398256/7d19077f-ef25-4d79-980b-cfa1775d928d">

sample/programs/ruby/simple.rb

@@ -5,6 +5,7 @@ def a
   for i in 0..100 do
     b
     c
+    e
   end
 end
 
@@ -28,8 +29,13 @@ def d
   prod
 end
 
-profile = StackProf.run(mode: :wall, raw: true) do
+def e
+  sleep 0.05
+end
+
+mode = (ARGV[0] || :wall).to_sym
+profile = StackProf.run(mode: mode, raw: true) do
   a
 end
 
-puts JSON.generate(profile)
+puts JSON.generate(profile)