Reuse existing small allocations if possible (#12335)

Those allocations are all small and insignificant in the grand scheme of
things and the optimizer may be able to resolve some of those but better
to be nice anyways.

Primarily inspired by the new
[`clippy::assigning_clones`](https://rust-lang.github.io/rust-clippy/master/index.html#/assigning_clones)

- **Avoid reallocs with `clone_from` in `nu-parser`**
- **Avoid realloc on assignment in `Stack`**
- **Fix `clippy::assigning_clones` in `nu-cli`**
- **Reuse allocations in `nu-explore` if possible**

crates/nu-protocol/src/engine/stack.rs

@@ -137,11 +137,11 @@ impl Stack {
     ) {
         // Do not clone the environment if it hasn't changed
         if self.env_vars.iter().any(|scope| !scope.is_empty()) {
-            self.env_vars = env_vars.to_owned();
+            env_vars.clone_into(&mut self.env_vars);
         }
 
         if !self.env_hidden.is_empty() {
-            self.env_hidden = env_hidden.to_owned();
+            self.env_hidden.clone_from(env_hidden);
         }
     }