nstack

nstack is a stack merkle datastructure for storing and accessing indexed values.

It builds on kelvin for database backing and merkle operations.

Usage example

```rust

use kelvin::{annotations::Cardinality, Blake2b};

let n: usize = 256; let mut nt = NStack::<_, Cardinality, Blake2b>::new();

for i in 0..n { nt.push(i).unwrap(); }

// get a mutable reference to the 128'th element

let element = &mut *nt.get_mut(128).unwrap().unwrap();

```

Structure

The general structure of the NStack is a 4-way splitting tree that is always populated from the "bottom left".

As new nodes are added, they get added to the right of the last leaf, creating a new root level on top when neccesary.

The leaves of the tree are always located at the same depth relative to the root.

Here's a representation, using width of 2 instead of 4 for easier comprehension.

``` containing 1 element:

[0, - ]

containing 3 elements:

[ *, * ] / \ [0, 1] [2, -]

containing 5 elements:

  [ *    ,    *  ]
   /           \

[ * , * ] [ * , - ] / \ / [0, 1] [2, 3] [ 4, - ] ```