Use only $nu.env.PWD for getting the current directory (#587)

* Use only $nu.env.PWD for getting current directory

Because setting and reading to/from std::env changes the global state
shich is problematic if we call `cd` from multiple threads (e.g., in a
`par-each` block).

With this change, when engine-q starts, it will either inherit existing
PWD env var, or create a new one from `std::env::current_dir()`.
Otherwise, everything that needs the current directory will get it from
`$nu.env.PWD`. Each spawned external command will get its current
directory per-process which should be thread-safe.

One thing left to do is to patch nu-path for this as well since it uses
`std::env::current_dir()` in its expansions.

* Rename nu-path functions

*_with is not *_relative which should be more descriptive and frees
"with" for use in a followup commit.

* Clone stack every each iter; Fix some commands

Cloning the stack each iteration of `each` makes sure we're not reusing
PWD between iterations.

Some fixes in commands to make them use the new PWD.

* Post-rebase cleanup, fmt, clippy

* Change back _relative to _with in nu-path funcs

Didn't use the idea I had for the new "_with".

* Remove leftover current_dir from rebase

* Add cwd sync at merge_delta()

This makes sure the parser and completer always have up-to-date cwd.

* Always pass absolute path to glob in ls

* Do not allow PWD a relative path; Allow recovery

Makes it possible to recover PWD by proceeding with the REPL cycle.

* Clone stack in each also for byte/string stream

* (WIP) Start moving env variables to engine state

* (WIP) Move env vars to engine state (ugly)

Quick and dirty code.

* (WIP) Remove unused mut and args; Fmt

* (WIP) Fix dataframe tests

* (WIP) Fix missing args after rebase

* (WIP) Clone only env vars, not the whole stack

* (WIP) Add env var clone to `for` loop as well

* Minor edits

* Refactor merge_delta() to include stack merging.

Less error-prone than doing it manually.

* Clone env for each `update` command iteration

* Mark env var hidden only when found in eng. state

* Fix clippt warnings

* Add TODO about env var reading

* Do not clone empty environment in loops

* Remove extra cwd collection

* Split current_dir() into str and path; Fix autocd

* Make completions respect PWD env var

crates/nu-path/src/tilde.rs

@@ -1,6 +1,6 @@
 use std::path::{Path, PathBuf};
 
-fn expand_tilde_with(path: impl AsRef<Path>, home: Option<PathBuf>) -> PathBuf {
+fn expand_tilde_with_home(path: impl AsRef<Path>, home: Option<PathBuf>) -> PathBuf {
     let path = path.as_ref();
 
     if !path.starts_with("~") {
@@ -27,7 +27,7 @@ fn expand_tilde_with(path: impl AsRef<Path>, home: Option<PathBuf>) -> PathBuf {
 /// Expand tilde ("~") into a home directory if it is the first path component
 pub fn expand_tilde(path: impl AsRef<Path>) -> PathBuf {
     // TODO: Extend this to work with "~user" style of home paths
-    expand_tilde_with(path, dirs_next::home_dir())
+    expand_tilde_with_home(path, dirs_next::home_dir())
 }
 
 #[cfg(test)]
@@ -37,17 +37,17 @@ mod tests {
     fn check_expanded(s: &str) {
         let home = Path::new("/home");
         let buf = Some(PathBuf::from(home));
-        assert!(expand_tilde_with(Path::new(s), buf).starts_with(&home));
+        assert!(expand_tilde_with_home(Path::new(s), buf).starts_with(&home));
 
         // Tests the special case in expand_tilde for "/" as home
         let home = Path::new("/");
         let buf = Some(PathBuf::from(home));
-        assert!(!expand_tilde_with(Path::new(s), buf).starts_with("//"));
+        assert!(!expand_tilde_with_home(Path::new(s), buf).starts_with("//"));
     }
 
     fn check_not_expanded(s: &str) {
         let home = PathBuf::from("/home");
-        let expanded = expand_tilde_with(Path::new(s), Some(home));
+        let expanded = expand_tilde_with_home(Path::new(s), Some(home));
         assert!(expanded == Path::new(s));
     }