Port simple heap for closure plugin

# Goal
Stabilize closure plugin.

## Overview
Built-in closure plugin doesn't manage memory for closures. I wrote a basic [simple-heap](https://github.com/ballercat/wasm-utils/blob/master/lib/simple-heap.js) in JavaScript a long time ago for emulated `malloc/free` calls. It should be trivial to port it to a walt file.

## Acceptance Criteria

- [ ] Implement actual `malloc` and `free`
- [ ] Write some tests

Start by reading the linked lib/simple-heap.js implementation and locating the built-in closure plugin's Walt source. Port the heap behavior there, implement malloc and free, and add tests that demonstrate both operations work as requested.

Written by the indexing model from the issue text.