Add verified µRust examples

[ethan-kharitonov]

Description of changes: Adds some more examples of µRust programs, verified using AutoCorrode.

• Number_Theory.thy: Fibonacci, GCD, extended GCD, Euler's totient, modular exponentiation, and a simple primality test
• FSet_Ref.thy: Reference-backed finite sets
• Stack.thy: Linked-list backed stack
• Graph.thy: Finite directed graphs with verified depth-first search

(Also adds .DS_Store to .gitignore. .DS_Store is a hidden macOS Finder settings file.)

Tested: isabelle build -d . Micro_Rust_Examples.

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[ethan-kharitonov]

Splitting into smaller PRs for easier review: #212, #213, #214. Will create another PR for the Graph example once FSet and Stack are merged.