Add a way to render trees and types as Scala code

* In a human-friendly way (for UIs, debugging)
* In a computer-friendly way (with all the excruciating details), notably for IDEs to make a compilable source out of tasty files

Coming from the discussion at the Scala Tooling Summit last week.
```[tasklist]
### Tasks
- [x] Render Types
- [x] Render Trees
- [ ] Render in a computer-friendly way
```
