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/impls/mod.rs

@@ -1,7 +1,7 @@
-mod btreemap;
+pub mod btreemap;
 mod deref;
-mod hashmap;
-mod index;
+pub mod hashmap;
+pub mod index;
 mod option;
 mod result;
 mod slice;