Create helix-stdx crate for stdlib extensions

helix-stdx is meant to carry extensions to the stdlib or low-level
dependencies that are useful in all other crates. This commit starts
with all of the path functions from helix-core and the CWD tracking that
lived in helix-loader.

The CWD tracking in helix-loader was previously unable to call the
canonicalization functions in helix-core. Switching to our custom
canonicalization code should make no noticeable difference though
since `std::env::current_dir` returns a canonicalized path with
symlinks resolved (at least on unix).

helix-stdx/src/env.rs

@@ -0,0 +1,48 @@
+use std::{
+    path::{Path, PathBuf},
+    sync::RwLock,
+};
+
+static CWD: RwLock<Option<PathBuf>> = RwLock::new(None);
+
+// Get the current working directory.
+// This information is managed internally as the call to std::env::current_dir
+// might fail if the cwd has been deleted.
+pub fn current_working_dir() -> PathBuf {
+    if let Some(path) = &*CWD.read().unwrap() {
+        return path.clone();
+    }
+
+    let path = std::env::current_dir()
+        .map(crate::path::normalize)
+        .expect("Couldn't determine current working directory");
+    let mut cwd = CWD.write().unwrap();
+    *cwd = Some(path.clone());
+
+    path
+}
+
+pub fn set_current_working_dir(path: impl AsRef<Path>) -> std::io::Result<()> {
+    let path = crate::path::canonicalize(path);
+    std::env::set_current_dir(&path)?;
+    let mut cwd = CWD.write().unwrap();
+    *cwd = Some(path);
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{current_working_dir, set_current_working_dir};
+
+    #[test]
+    fn current_dir_is_set() {
+        let new_path = dunce::canonicalize(std::env::temp_dir()).unwrap();
+        let cwd = current_working_dir();
+        assert_ne!(cwd, new_path);
+
+        set_current_working_dir(&new_path).expect("Couldn't set new path");
+
+        let cwd = current_working_dir();
+        assert_eq!(cwd, new_path);
+    }
+}