Complete ImmutableString.

RELEASES.md

@@ -4,8 +4,8 @@ Rhai Release Notes
 Version 0.14.2
 ==============
 
-Regression
-----------
+Regression fix
+--------------
 
 * Do not optimize script with `eval_expression` - it is assumed to be one-off and short.
 
@@ -18,6 +18,10 @@ Breaking changes
 * Default maximum limit on levels of nested function calls is fine-tuned and set to a different value.
 * Some operator functions are now built in (see _Speed enhancements_ below), so they are available even
   under `Engine::new_raw`.
+* Strings are now immutable. The type `rhai::ImmutableString` is used instead of `std::string::String`.
+  This is to avoid excessive cloning of strings.  All native-Rust functions taking string parameters
+  should switch to `rhai::ImmutableString` (which is either `Rc<String>` or `Arc<String>` depending on
+  whether the `sync` feature is used).
 
 New features
 ------------
@@ -35,6 +39,8 @@ Speed enhancements
   significant speed-up.
 * Implementations of common operators for standard types are removed from the `ArithmeticPackage` and `LogicPackage`
   (and therefore the `CorePackage`) because they are now always available, even under `Engine::new_raw`.
+* Operator-assignment statements (e.g. `+=`) are now handled directly and much faster.
+* Strings are now _immutable_ and use the `rhai::ImmutableString` type, eliminating large amounts of cloning.
 
 
 Version 0.14.1