Use `PtrMetadata` instead of `Len` in slice drop shims I tried to do a bigger change in #134297 which didn't work, so here's the part I really wanted: Removing another use of `Len`, in favour of `PtrMetadata`. Split into two commits where the first just adds a test, so you can look at the second commit to see how the drop shim for an array changes with this PR. Reusing the same reviewer from the last one: r? BoxyUwU