Auto merge of #51814 - MajorBreakfast:local-task-obj, r=cramertj

Add `LocalTaskObj` to `core::task`

- Splits `libcore/task.rs` into submodules
- Adds `LocalTaskObj` and `SpawnLocalObjError` (-> [Commit for this](https://github.com/rust-lang/rust/commit/433e6b31a75eea5ce45493acc63eae462d740338))

Note: To make reviewing easy, both actions have their own commit

r? @cramertj