Memory pool/reuse

[Dentosal]

Used by https://github.com/FuelLabs/fuel-vm/pull/732

Adds a method to reset memory to the original state.

Also adds a memory pool to allow reusing memory instances, and use that for all VM initializations.

Checklist

• [x] Breaking changes are clearly marked as such in the PR description and changelog
• [x] New behavior is reflected in tests

Before requesting review

• [x] I have reviewed the code myself