eval-with-cursor

Rewrite eval to use the cursor. Read digit, advance, read op, advance, read digit, advance. The same four checks should still pass.

fn eval(input: []const u8) i64 {
    source = input;
    pos = 0;
    const a: i64 = digit(cur());
    pos += 1;
    const op: u8 = cur();
    pos += 1;
    const b: i64 = digit(cur());
    pos += 1;
    return switch (op) {
        '+' => a + b,
        '-' => a - b,
        '*' => a * b,
        '/' => @divTrunc(a, b),
        else => 0,
    };
}

Same answers. But now we can move.