backend: Default nesting levels to 2

[raffenet]

Pull Request Description

This significantly reduces the build time and binary size, which is critical for large-scale.

Expected Impact

Reduced binary size. Performance for deeply nested datatypes may suffer, but that is a secondary concern.

Author Checklist

• Reference appropriate issues (with "Fixes" or "See" as appropriate)
• Commits are self-contained and do not do two things at once
• Commit message is of the form: module: short description and follows good practice
• Add comments such that someone without knowledge of the code could understand
• Have read and agree to the Yaksa CLA terms (https://github.com/pmodels/yaksa/wiki/Yaksa-Contributor-License-Agreement)