Add a simple way to manually generate Pair

[gjvnq]

Hi, I am writing some unit tests for a function that takes Pair<'_, Rule> as an input. The problem is that I am forced to call the parser to generate those pairs, thus making my unit tests not that independent.

Example: I have

let input = EbolParser::parse(Rule::Integer, "42").unwrap();
println!("{:?}", input); // Prints: [Pair { rule: Integer, span: Span { str: "42", start: 0, end: 2 }, inner: [] }]
my_function(input.peek());

But I would like to have something like:

let input = Pair { rule: Rule::Integer, span: Span { str: "42", start: 0, end: 2 }, inner: [] };
my_function(input);