Fix stores with vec index method and expose opaque writer types (#4684)

* Use a different indexing method for each data structure

* add a get method for slices

* Rename trait for clarity and simplicity

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Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

packages/stores/src/lib.rs

@@ -7,6 +7,7 @@
 mod impls;
 mod store;
 mod subscriptions;
+pub use impls::*;
 pub use store::*;
 pub mod scope;