stricter task isolation for incremental compilation tasks

[nikomatsakis]

NB: Part of the roadmap issue on incremental compilation.

The current scheme of task isolation allows leaks anywhere (e.g., DepGraph::with_task and friends). We should move to a scheme where the context is a closure that uses an autotrait to forbid shared, mutable state (basically, forbid UnsafeCell) unless that state is "tracked" -- meaning that its reads and writes are accounted for in the dependency graph. The metadata code uses a scheme vaguely like this.

This will probably be a transition process, starting with an initial PR to construct the "opt-in" mechanism and then replacing existing uses in a series of smaller PRs.

[nikomatsakis]

I've decided it would be better to remove tasks, as described in #40746