ChainMap

This library provides a chain of HashMaps with interior mutability of each intermediate HashMap. The HashMaps are reference-counted, meaning that it is possible to create a tree of layers of HashMaps and not just a single chain.

The higher maps in the tree (close to the leaves) have higher priority.

One possible use case is for the management of nested scopes.

An example from the appropriate section of the Book: 15. Scoping rules - RAII

```rust fn create_box() { // CreateBoxScope let _box1 = Box::new(3i32); }

fn main() { // MainScope let _box2 = Box::new(5i32); { // NestedScope let box3 = Box::new(4i32); }

for i in 0u32..1_000 {
    // LoopScope<i>
    create_box();
}

} ```

Could be represented as MainScope["_box2" => 5i32] ├── NestedScope["_box3" => 4i32] ├── LoopScope0[] │ └── CreateBoxScope["_box1" => 3i32] ├── LoopScope1[] │ └── CreateBoxScope["_box1" => 3i32] │ ... └── LoopScope1000[] └── CreateBoxScope["_box1" => 3i32] Where each [ $( $key => $value ),* ] is a level of a tree of ChainMaps built on the previous one. The rules for which map entries are accessible from a certain level of the ChainMap tree are exactly the same as how they would be for the corresponding scopes.