Reuse generator outputs

[jhenninger]

Right now the generator function is run again before each part, even if all parts request exactly the same input. This wastes computation time because the puzzle input has to be parsed and transformed at least twice.

It would be preferable if the output of the generator function could be reused, especially if the solver functions only take immutable references, like the examples in the readme.

For example, if the generator returns a Vec,

#[aoc_generator(day2)]
pub fn input_generator(input: &str) -> Vec<Gift> {
    // ...
}

and the solvers take an immutable slice or another reference type,

#[aoc(day2, part1)]
pub fn solve_part1(input: &[Gift]) -> u32 {
    // ...
}

#[aoc(day2, part2)]
pub fn solve_part2(input: &[Gift]) -> u32 {
    // ...
}

it should be possible to use the same Vec returned by the generator for both solvers.