Fix mismatched_lifetime_syntaxes warnings

[anthonyroussel]

Fixes the mismatched lifetime syntaxes warnings.

Rust 1.89 includes a new lint for mismatched lifetime syntaxes:

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/channel/message.rs:153:25
    |
153 |     fn extract(message: &str) -> (String, SplitWhitespace) {
    |                         ^^^^              ^^^^^^^^^^^^^^^ the same lifetime is hidden here
    |                         |
    |                         the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
153 |     fn extract(message: &str) -> (String, SplitWhitespace<'_>) {
    |                                                          ++++