miri: add test checking that aggregate assignments reset memory to uninit Also, `write_aggregate` is really just a helper for evaluating `Aggregate` rvalues, so it should be in `step.rs`, not `place.rs`. Also factor out `Repeat` rvalues into their own function while we are at it. r? `@saethlin` Fixes https://github.com/rust-lang/miri/issues/3195