bvnierop advent-of-code-rust .cursorrules file for Rust

# Conventions

When writing code, stick to the following conventions.

## Programming language
The programming language used shall be Rust.

## Conventions
- Write idiomatic Rust
- Write clear, readable and verbose code over terse, dense and concise code
- Public functions should have documentation
- Public functions should have doc tests
- Prefer using existing libraries over implementing features
- Embrace SOLID principles
- Keep cognitive complexity low
- Only use mocks at system boundaries (filesystem, network, time, etc)