rs-01-rle

# Run-length encoding

Implement the library file **`src/lib.rs`** exposing:

```rust
pub fn run_length_encode(input: &str) -> String
```

Encode consecutive runs of the same character as `<char><count>`. A single character still
gets a count of `1`.

- `run_length_encode("aaabbc")` → `"a3b2c1"`
- `run_length_encode("")` → `""`
- `run_length_encode("abc")` → `"a1b1c1"`

Tests live in `tests/` and call it as `challenge::run_length_encode`.

use challenge::run_length_encode;

#[test]
fn basic_runs() {
    assert_eq!(run_length_encode("aaabbc"), "a3b2c1");
}

#[test]
fn empty() {
    assert_eq!(run_length_encode(""), "");
}

#[test]
fn all_singles() {
    assert_eq!(run_length_encode("abc"), "a1b1c1");
}

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```rust
pub fn run_length_encode(input: &str) -> String {
    let mut result = String::new();
    
    if input.is_empty() {
        return result;
    }

    let mut chars = input.chars().peekable();
    
    while let Some(current_char) = chars.next() {
        let mut count = 1;
        
        while chars.peek() == Some(&current_char) {
            chars.next();
            count += 1;
        }
        
        result.push(current_char);
        result.push_str(&count.to_string());
    }
    
    result
}
```