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-command/src/strings/detect_columns.rs

@@ -226,7 +226,7 @@ fn detect_columns(
                 .iter()
                 .take(end_index)
                 .skip(start_index)
-                .map(|v| v.coerce_string().unwrap_or_default())
+                .map(|v| v.coerce_str().unwrap_or_default())
                 .join(" ");
             let binding = Value::string(combined, Span::unknown());
             let last_seg = vals.split_off(end_index);

crates/nu-command/src/strings/split/chars.rs

@@ -124,7 +124,7 @@ fn split_chars_helper(v: &Value, name: Span, graphemes: bool) -> Value {
         Value::Error { error, .. } => Value::error(*error.clone(), span),
         v => {
             let v_span = v.span();
-            if let Ok(s) = v.coerce_string() {
+            if let Ok(s) = v.coerce_str() {
                 Value::list(
                     if graphemes {
                         s.graphemes(true)

crates/nu-command/src/strings/split/column.rs

@@ -148,7 +148,7 @@ fn split_column_helper(
     collapse_empty: bool,
     head: Span,
 ) -> Vec<Value> {
-    if let Ok(s) = v.coerce_string() {
+    if let Ok(s) = v.coerce_str() {
         let split_result: Vec<_> = separator
             .split(&s)
             .filter(|x| !(collapse_empty && x.is_empty()))

crates/nu-command/src/strings/split/list.rs

@@ -160,7 +160,7 @@ enum Matcher {
 impl Matcher {
     pub fn new(regex: bool, lhs: Value) -> Result<Self, ShellError> {
         if regex {
-            Ok(Matcher::Regex(Regex::new(&lhs.coerce_string()?).map_err(
+            Ok(Matcher::Regex(Regex::new(&lhs.coerce_str()?).map_err(
                 |e| ShellError::GenericError {
                     error: "Error with regular expression".into(),
                     msg: e.to_string(),
@@ -180,7 +180,7 @@ impl Matcher {
     pub fn compare(&self, rhs: &Value) -> Result<bool, ShellError> {
         Ok(match self {
             Matcher::Regex(regex) => {
-                if let Ok(rhs_str) = rhs.coerce_string() {
+                if let Ok(rhs_str) = rhs.coerce_str() {
                     regex.is_match(&rhs_str)
                 } else {
                     false

crates/nu-command/src/strings/split/row.rs

@@ -152,7 +152,7 @@ fn split_row_helper(v: &Value, regex: &Regex, max_split: Option<usize>, name: Sp
         v => {
             let v_span = v.span();
 
-            if let Ok(s) = v.coerce_string() {
+            if let Ok(s) = v.coerce_str() {
                 match max_split {
                     Some(max_split) => regex
                         .splitn(&s, max_split)

crates/nu-command/src/strings/split/words.rs

@@ -158,7 +158,7 @@ fn split_words_helper(v: &Value, word_length: Option<usize>, span: Span, graphem
         Value::Error { error, .. } => Value::error(*error.clone(), v_span),
         v => {
             let v_span = v.span();
-            if let Ok(s) = v.coerce_string() {
+            if let Ok(s) = v.coerce_str() {
                 // let splits = s.unicode_words();
                 // let words = trim_to_words(s);
                 // let words: Vec<&str> = s.split_whitespace().collect();