feat: shortcircuit merging two of the same value during flattening

[TomAFrench]

Description

Problem*

Helps with #4688

Summary*

This PR short circuits merge_numeric_values if it's passed two copies of the same value.

I've also replaced some clones in the flattening pass with references.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

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

[TomAFrench]

Looks like I had an old version of nargo which I was initially comparing against.