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

---------

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

packages/stores/src/impls/btreemap.rs

@@ -1,3 +1,5 @@
+//! Additional utilities for `BTreeMap` stores.
+
 use std::{borrow::Borrow, collections::BTreeMap, hash::Hash, iter::FusedIterator};
 
 use crate::{store::Store, ReadStore};