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-cmd-lang/src/example_support.rs

@@ -270,7 +270,7 @@ impl<'a> std::fmt::Debug for DebuggableValue<'a> {
                 write!(f, "Binary({:?})", val)
             }
             Value::CellPath { val, .. } => {
-                write!(f, "CellPath({:?})", val.into_string())
+                write!(f, "CellPath({:?})", val.to_string())
             }
             Value::CustomValue { val, .. } => {
                 write!(f, "CustomValue({:?})", val)