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`.

crates/nu_plugin_query/src/query_json.rs

@@ -8,8 +8,8 @@ pub fn execute_json_query(
     input: &Value,
     query: Option<Spanned<String>>,
 ) -> Result<Value, LabeledError> {
-    let input_string = match &input.coerce_string() {
-        Ok(s) => s.clone(),
+    let input_string = match input.coerce_str() {
+        Ok(s) => s,
         Err(e) => {
             return Err(LabeledError {
                 span: Some(call.head),

crates/nu_plugin_query/src/query_web.rs

@@ -332,15 +332,15 @@ mod tests {
             Span::test_data(),
         );
         let out = item
-            .as_list()
+            .into_list()
             .unwrap()
-            .iter()
+            .into_iter()
             .map(|matches| {
                 matches
-                    .as_list()
+                    .into_list()
                     .unwrap()
-                    .iter()
-                    .map(|text_nodes| text_nodes.coerce_string().unwrap())
+                    .into_iter()
+                    .map(|text_nodes| text_nodes.coerce_into_string().unwrap())
                     .collect::<Vec<String>>()
             })
             .collect::<Vec<Vec<String>>>();

crates/nu_plugin_query/src/query_xml.rs

@@ -21,7 +21,7 @@ pub fn execute_xpath_query(
     };
 
     let xpath = build_xpath(query_string, span)?;
-    let input_string = input.coerce_string()?;
+    let input_string = input.coerce_str()?;
     let package = parser::parse(&input_string);
 
     if package.is_err() {