fix: rename q c field to constant in expression struct

[avorylli]

Description

Problem*

Resolves TODO comment in acir/src/native_types/expression/mod.rs about renaming the q_c field.

Summary*

While exploring the codebase, I found a TODO comment suggesting to rename the q_c field to constant in the Expression struct and implemented this change.

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.

[github-actions]

Thank you for your contribution to the Noir language.

Please do not force push to this branch after the Noir team have started review of this PR. Doing so will only delay us merging your PR as we will need to start the review process from scratch.

Thanks for your understanding.