Auto merge of #30533 - nikomatsakis:fulfillment-tree, r=aturon

This PR introduces an `ObligationForest` data structure that the fulfillment context can use to track what's going on, instead of the current flat vector. This enables a number of improvements:

1. transactional support, at least for pushing new obligations
2. remove the "errors will be reported" hack -- instead, we only add types to the global cache once their entire subtree has been proven safe. Before, we never knew when this point was reached because we didn't track the subtree.
   - this in turn allows us to limit coinductive reasoning to structural traits, which sidesteps #29859
3. keeping the backtrace should allow for an improved error message, where we give the user full context
    - we can also remove chained obligation causes

This PR is not 100% complete. In particular:

- [x] Currently, types that embed themselves like `struct Foo { f: Foo }` give an overflow when evaluating whether `Foo: Sized`. This is not a very user-friendly error message, and this is a common beginner error. I plan to special-case this scenario, I think.
- [x] I should do some perf. measurements. (Update: 2% regression.)
- [x] More tests targeting #29859
- [ ] The transactional support is not fully integrated, though that should be easy enough.
- [ ] The error messages are not taking advantage of the backtrace.

I'd certainly like to do 1 through 3 before landing, but 4 and 5 could come as separate PRs.

r? @aturon // good way to learn more about this part of the trait system
f? @arielb1 // already knows this part of the trait system :)

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