Add `Profiler::finish_into_string(self) -> String`.

[finnbear]

Thanks for making this. I successfully integrated it into my server such that I can produce heap profiles on demand. However, I noticed the following issues:

• I had to use a #[doc(hidden)] function
  • And I had to call std::mem::forget too to avoid a double-drop (your tests used ManuallyDrop)
• It produces unwanted stderr output

This PR addresses these issues, at the cost of making the API a bit more confusing (you can set an output path and then have it be ignored when you finish into a String).