Split PySparseObservable off SparseObservable (#13595)

* Split ``PySparseObservable`` or ``SparseObservable``

* reno and docs

* rm Arc<RwLock> for SparseTerm

* add Into(Py) for SparseObservable

* rm Rust-side PySparseObservable.clone

* smaller fixes

- avoid using &Vec
- add unsafe to mutable indices/boundaries
- rm upgrade reno

* move back into 1 file

* Bring trait impls closer to structs

* review comments

- use Arc<> instead of Py<> in ArrayView
- py_sparse.. -> sparse_

* Tidy up whitespace

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Co-authored-by: Jake Lishman <[email protected]>