chore: fix up nits

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR fixes up a few issues:

• noir-gates-diff commit is old (we've done some updates to this recently)
• we're currently fuzzing a circuit with no inputs so I've converted that into a regular test.
• There were a fair few visibility issues in the tests so I've restructured things so we don't need to make the sha256 internals pub(crate)
• Clippy is angry about some of the rust code.

I've also updated authors to just be the noir team generally as the implementation of this lib is pre-existing.

Additional Context

PR Checklist*

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

[jtriley2p]

lgtm; shipp