Implement Display for CellPath (#11023)

# Description
Because `CellPath::into_string` takes a borrowed `self`, I renamed it to
`to_string` to follow Rust [API
guidelines](https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv).
This then triggered the clippy lint
[inherent_to_string](https://rust-lang.github.io/rust-clippy/master/index.html#/inherent_to_string),
which is... correct! The current `CellPath::into_string` is being used
as if it were the `Display` implementation for `CellPath`.

# User-Facing Changes
Breaking API change for `nu-protocol`, since `CellPath::into_string` was
removed.

crates/nu-command/src/database/commands/into_sqlite.rs

@@ -262,7 +262,7 @@ fn nu_value_to_string(value: Value, separator: &str) -> String {
         Value::Nothing { .. } => String::new(),
         Value::Error { error, .. } => format!("{error:?}"),
         Value::Binary { val, .. } => format!("{val:?}"),
-        Value::CellPath { val, .. } => val.into_string(),
+        Value::CellPath { val, .. } => val.to_string(),
         Value::CustomValue { val, .. } => val.value_string(),
         Value::MatchPattern { val, .. } => format!("{:?}", val),
     }

crates/nu-command/src/debug/explain.rs

@@ -257,7 +257,7 @@ pub fn debug_string_without_formatting(value: &Value) -> String {
         Value::Nothing { .. } => String::new(),
         Value::Error { error, .. } => format!("{error:?}"),
         Value::Binary { val, .. } => format!("{val:?}"),
-        Value::CellPath { val, .. } => val.into_string(),
+        Value::CellPath { val, .. } => val.to_string(),
         Value::CustomValue { val, .. } => val.value_string(),
         Value::MatchPattern { val, .. } => format!("{:?}", val),
     }

crates/nu-command/src/filters/flatten.rs

@@ -163,7 +163,7 @@ fn flat_value(columns: &[CellPath], item: Value, all: bool) -> Vec<Value> {
             let mut inner_table = None;
 
             for (column_index, (column, value)) in val.into_iter().enumerate() {
-                let column_requested = columns.iter().find(|c| c.into_string() == column);
+                let column_requested = columns.iter().find(|c| c.to_string() == column);
                 let need_flatten = { columns.is_empty() || column_requested.is_some() };
                 let span = value.span();
 

crates/nu-command/src/filters/select.rs

@@ -254,7 +254,7 @@ fn select(
                                 //FIXME: improve implementation to not clone
                                 match input_val.clone().follow_cell_path(&path.members, false) {
                                     Ok(fetcher) => {
-                                        record.push(path.into_string().replace('.', "_"), fetcher);
+                                        record.push(path.to_string().replace('.', "_"), fetcher);
                                         if !columns_with_value.contains(&path) {
                                             columns_with_value.push(path);
                                         }
@@ -284,7 +284,7 @@ fn select(
                             // FIXME: remove clone
                             match v.clone().follow_cell_path(&cell_path.members, false) {
                                 Ok(result) => {
-                                    record.push(cell_path.into_string().replace('.', "_"), result);
+                                    record.push(cell_path.to_string().replace('.', "_"), result);
                                 }
                                 Err(e) => return Err(e),
                             }
@@ -309,7 +309,7 @@ fn select(
                         //FIXME: improve implementation to not clone
                         match x.clone().follow_cell_path(&path.members, false) {
                             Ok(value) => {
-                                record.push(path.into_string().replace('.', "_"), value);
+                                record.push(path.to_string().replace('.', "_"), value);
                             }
                             Err(e) => return Err(e),
                         }

crates/nu-command/src/formats/to/text.rs

@@ -146,7 +146,7 @@ fn local_into_string(value: Value, separator: &str, config: &Config) -> String {
         Value::Nothing { .. } => String::new(),
         Value::Error { error, .. } => format!("{error:?}"),
         Value::Binary { val, .. } => format!("{val:?}"),
-        Value::CellPath { val, .. } => val.into_string(),
+        Value::CellPath { val, .. } => val.to_string(),
         Value::CustomValue { val, .. } => val.value_string(),
         Value::MatchPattern { val, .. } => format!("{:?}", val),
     }