Performance improvements to CPU profile code

After having taken a look at the cpu profiling code in DevTools, we found:

* It unnecessarily encodes json maps and decodes them again: [devtools/pull/7916](https://github.com/flutter/devtools/pull/7916)
* It gets `Stream */>` from the websocket and then json decodes strings
=> It would be more efficient to avoid going from utf-8 to string and then to json and instead directly from utf8->json
=> This would probably require some refactoring in various packages, so maybe not easily doable?
* Way too many conversions
* Bytes->String in websocket layer
* String->JSON in `package:vm_service layer`
* JSON -> service objects in `package:vm_service` layer (e.g. `vm_service.CpuSamples`, `vm_service.CpuSample`)
* service objects -> `_CpuProfileTimelineTree` tree structure
=> Uses expandos (!!) to map service objects back to tree nodes
* `_CpuProfileTimelineTree` -> JSON "traceObject" (very expensive operation!)
* Going from JSON "traceObject" back to normal objects in `CpuProfileData.fromJson` which builds json maps with strings and `CpuStackFrame` as values

It seems there's too many conversions from one data structure to another and the representations used for encoding this profiling information doesn't seem very efficient either.