Add Cow<'a, N, LenT> type for heapless strings

- Introduced a clone-on-write (Cow) enum for heapless strings.
- Supports borrowed (&StringView<LenT>) and owned (String<N, LenT>) variants.
- Provides to_owned(), as_str(), is_borrowed(), and is_owned() helper methods.
- Implements From for StringView and String.
- Implements Borrow<str> for ergonomic use.
- Fully bounded by LenT: LenType to match heapless string types.

Author: KushalMeghani1644

src/cow.rs

@@ -0,0 +1,56 @@
+#![cfg(feature = "alloc")]
+
+extern crate alloc;
+
+use crate::len_type::LenType;
+use crate::{CapacityError, String, StringView};
+use core::borrow::Borrow;
+
+/// A clone-on-write string type for heapless
+#[derive(Debug)]
+pub enum Cow<'a, const N: usize, LenT: LenType = usize> {
+    Borrowed(&'a StringView<LenT>),
+    Owned(String<N, LenT>),
+}
+
+impl<'a, const N: usize, LenT: LenType> Cow<'a, N, LenT> {
+    pub fn to_owned(&self) -> String<N, LenT> {
+        match self {
+            Cow::Borrowed(sv) => String::from(sv),
+            Cow::Owned(s) => s.clone(),
+        }
+    }
+
+    pub fn as_str(&self) -> &str {
+        match self {
+            Cow::Borrowed(sv) => sv.as_str(),
+            Cow::Owned(s) => s.as_str(),
+        }
+    }
+
+    pub fn is_borrowed(&self) -> bool {
+        matches!(self, Cow::Borrowed(_))
+    }
+
+    pub fn is_owned(&self) -> bool {
+        matches!(self, Cow::Owned(_))
+    }
+}
+
+impl<'a, const N: usize, LenT: LenType> From<&'a StringView<LenT>> for Cow<'a, N, LenT> {
+    fn from(sv: &'a StringView<LenT>) -> Self {
+        Cow::Borrowed(sv)
+    }
+}
+
+impl<const N: usize, LenT: LenType> From<String<N, LenT>> for Cow<'_, N, LenT> {
+    fn from(s: String<N, LenT>) -> Self {
+        Cow::Owned(s)
+    }
+}
+
+impl<'a, const N: usize, LenT: LenType> Borrow<str> for Cow<'a, N, LenT> {
+    fn borrow(&self) -> &str {
+        self.as_str()
+    }
+}

src/lib.rs

@@ -166,6 +166,7 @@ pub use vec::{Vec, VecView};
 mod test_helpers;
 
 pub mod c_string;
+pub mod cow;
 pub mod deque;
 pub mod history_buf;
 pub mod index_map;