fix(brillig): expand memory with zeroes on store

[sirasistant]

Description

Problem*

Brillig VM memory was fixed on initialization, which requires to always initialize it with an empty memory of the maximum size allowed.

Summary*

This PR expands the memory dynamically on store with zeroes. In the future we can add a feature of passing a maximum memory on VM creation to limit VM memory for applications where brillig is going to be verified with constrained resources, such as in a VM circuit.

Example

Before:

After:

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.