Clean up mill.eval even more

[lihaoyi]

1. Terminal goes in its own file
2. Split out EvaluatorCore (which contains only evaluation-related stuff) from EvaluatorImpl (which contains some user-facing helpers)
3. Split out GroupEvaluator from EvaluatorCore
4. Split out Plan, ThreadNumberer, JsonArrayLogger
5. Unify implementation of mill-profile.json and mill-par-profile.json (renamed mill-chrome-profile.json), so they both perform streaming as-evaluation-progresses logging in a thread-safe manner (should fix Randomly ConcurrentModificationException #2540)
6. Add dependencies to mill-profile.json, making it more useful since you can now trace up and down the build graph without having to run mill inspect (which would wipe out the mill-profile.json you were just looking at!)