Fixup lifetimes of PrattParser

[matanui159]

Currently the typings require the passed in callbacks to have the lifetime 'i, and thus any values it reference must also have the same lifetime. If these callbacks return a value with lifetime 'i, the borrow checker incorrectly assumes that those referenced values must also outlive the return value.

For example this code:

fn test_pratt<'a>(pairs: Pairs<'a, Rule>, message: &str) -> &'a str {
    PRATT_PARSER
        .map_primary(|pair| {
            // println!("{message}");
            pair.as_str()
        })
        .map_infix(|lhs, pair, rhs| {
            println!("{message}");
            pair.as_str()
        })
        .parse(pairs)
}

fails to compile with this error:

error[E0621]: explicit lifetime required in the type of `message`
   --> src/compiler/expression.rs:285:5
    |
284 |   fn test_pratt<'a>(pairs: Pairs<'a, Rule>, message: &str) -> &'a str {
    |                                                      ---- help: add explicit lifetime `'a` to the type of `message`: `&'a str`
285 | /     PRATT_PARSER
286 | |         .map_primary(|pair| {
287 | |             // println!("{message}");
288 | |             pair.as_str()
...   |
293 | |         })
294 | |         .parse(pairs)
    | |_____________________^ lifetime `'a` required

This is a small PR that adds a new lifetime to PrattParserMap so the callbacks can have a separate lifetime. This fixes the above issue.

[tomtau]

Makes sense, thanks for the PR!