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.

先阅读 DevTools 中的 CPU 分析流程，包括 package:vm_service、_CpuProfileTimelineTree 和 CpuProfileData.fromJson。查看 devtools/pull/7916，并跟踪列出的每一次转换：从 websocket bytes 经过 service objects 到 traceObject 输出。Done 要求就减少不必要的转换达成共识并进行测量，同时定义重构范围。

由索引模型根据 Issue 内容生成。