Optimization of tilde expansion (#9709)

* Use next and avoid a redundant prefix strip

* Avoid allocations

Especially when `expand_tilde` is claled on a path
that doesn't contain a tilde.

* Add a test

* Use Into<Cow<…>>

* Put the expand_tilde test at the end of the file

* Remove unused importsw

helix-loader/src/lib.rs

@@ -53,7 +53,7 @@ fn prioritize_runtime_dirs() -> Vec<PathBuf> {
     rt_dirs.push(conf_rt_dir);
 
     if let Ok(dir) = std::env::var("HELIX_RUNTIME") {
-        let dir = path::expand_tilde(dir);
+        let dir = path::expand_tilde(Path::new(&dir));
         rt_dirs.push(path::normalize(dir));
     }