Macro solution

macro_rules! solution {
    ($day:expr, $parser:expr, $solver:expr) => { ... };
}

Declare a new solution:

• $day is the day of the exercise this solution solves. This is used to download the input file automatically.
• $parser is an instance of crate::parser::Parsing. (See [failable_parser!()] or [parser!()])
• $solver is an instance of crate::solution::Solver. (See [solver!()])