fix: Make Vec::get accept immutable Vecs

[jfecher]

Description

Problem*

Resolves #2775

Summary*

Vec::get used to require a mutable Vec since we did not automatically dereference &mut T -> T for objects of method calls before PR #2581. Now that this PR is merged, it can be immutable.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.
    • The type signature of Vec::get will need to be updated

Additional Context

PR Checklist*

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