one-stack-vec 0.4.0

OneStackVec could contain any number of item like `Vec`, and it does heap allocation only when it contains more than one item.
Documentation 
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One-Stack-Vec
===

## Overview
OneStackVec is simple data structure which contains any number of items.
It does not heap allocation until there is one or zero item.

## How to use
```rust
// Initialize
let mut items = OneStackVec::new();

// Adding
items.add(42);

// Getting
assert_eq!(Some(&42), items.get(0));

// Removing
assert_eq!(Some(&42), items.remove(0));
assert_eq!(None, items.get(0));
```