match lowering: test one or pattern at a time
This is a bit more opinionated than the previous PRs. On the face of it this is less efficient and more complex than before, but I personally found the loop that digs into `leaf_candidates` on each iteration very confusing. Instead this does "generate code for this or-pattern" then "generate further code for each branch if needed" in two steps.
Incidentally this way we don't _require_ or patterns to be sorted at the end. It's still an important optimization but I find it clearer to not rely on it for correctness.
r? `@matthewjasper`
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