Add Value::coerce_str (#11885)

# Description
Following #11851, this PR adds one final conversion function for
`Value`. `Value::coerce_str` takes a `&Value` and converts it to a
`Cow<str>`, creating an owned `String` for types that needed converting.
Otherwise, it returns a borrowed `str` for `String` and `Binary`
`Value`s which avoids a clone/allocation. Where possible, `coerce_str`
and `coerce_into_string` should be used instead of `coerce_string`,
since `coerce_string` always allocates a new `String`.

src/test_bins.rs

@@ -325,9 +325,9 @@ pub fn nu_repl() {
 
         if let Some(cwd) = stack.get_env_var(&engine_state, "PWD") {
             let path = cwd
-                .coerce_string()
+                .coerce_str()
                 .unwrap_or_else(|err| outcome_err(&engine_state, &err));
-            let _ = std::env::set_current_dir(path);
+            let _ = std::env::set_current_dir(path.as_ref());
             engine_state.add_env_var("PWD".into(), cwd);
         }
     }