fix: remove function which was ignoring compiler warning to not do that

[TomAFrench]

Description

Problem*

Resolves

Summary*

Also fixes a bunch of visibility issues and stuff.

Additional Context

PR Checklist*

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

[iAmMichaelConnor]

Removing __batch_invert_slice broke the bigcurve lib (when updating that lib to the latest version of noir) in such a way that I'm not even sure how to fix it, since slices are so pervasive in bigcurve.