Fix warnings about elided tsk crate lifetimes

[panhania]

Without this we are seeing warnings like:

warning: hiding a lifetime that's elided elsewhere is confusing
   --> crates/tsk/src/lib.rs:178:21                                   
    |                                                                                                                                        
178 |     pub fn open_dir(&self, path: &Path) -> Result<Directory> {                                                                         
    |                     ^^^^^                         --------- 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                                                                                                                
    |                                                                 
178 |     pub fn open_dir(&self, path: &Path) -> Result<Directory<'_>> {
    |                                                            ++++