* It is now possible to mutate the first argument of a module-qualified function call when the argument is a simple variable (but not a module constant).
* The tuple `ParseError` now exposes the internal fields and the `ParseError::error_type` and `ParseError::position` methods are removed. The first tuple field is the `ParseErrorType` and the second tuple field is the `Position`.
* Indexers are now split into getters and setters (which now support updates). The API is split into `Engine::register_indexer_get` and `Engine::register_indexer_set` with `Engine::register_indexer_get_set` being a shorthand. Similarly, `Module::set_indexer_get_fn` and `Module::set_indexer_set_fn` are added.
*`Engine:register_fn` and `Engine:register_result_fn` accepts functions that take parameters of type `&str` (immutable string slice), which maps directly to `ImmutableString`. This is to avoid needing wrappers for functions taking string parameters.
This version uses immutable strings (`ImmutableString` type) and built-in operator functions (e.g. `+`, `>`, `+=`) to improve speed, plus some bug fixes.
* 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).
* Native Rust functions registered with the `Engine` also mutates the first argument when called in normal function-call style (previously the first argument will be passed by _value_ if not called in method-call style). Of course, if the first argument is a calculated value (e.g. result of an expression), then mutating it has no effect, but at least it is not cloned.
* Some built-in methods (e.g. `len` for string, `floor` for `FLOAT`) now have _property_ versions in addition to methods to simplify coding.
* Common operators (e.g. `+`, `>`, `==`) now call into highly efficient built-in implementations for standard types (i.e. `INT`, `FLOAT`, `bool`, `char`, `()` and `ImmutableString`) if not overridden by a registered function. This yields a 5-10% speed benefit depending on script operator usage. Scripts running tight loops will see significant speed-up.
* Common assignment operators (e.g. `+=`, `%=`) now call into highly efficient built-in implementations for standard types (i.e. `INT`, `FLOAT`, `bool`, `char`, `()` and `ImmutableString`) if not overridden by a registered function.
* 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`.
* For Native Rust functions taking a first `&mut` parameter, the first argument is passed by reference instead of by value, even if not called in method-call style. This allows many functions declared with `&mut` parameter to avoid excessive cloning. For example, if `a` is a large array, getting its length in this manner: `len(a)` used to result in a full clone of `a` before taking the length and throwing the copy away. Now, `a` is simply passed by reference, avoiding the cloning altogether.
* A custom hasher simply passes through `u64` keys without hashing to avoid function call hash keys (which are by themselves `u64`) being hashed twice.
* Modules and _module resolvers_ allow loading external scripts under a module namespace. A module can contain constant variables, Rust functions and Rhai functions.
* A new API, `Engine::compile_scripts_with_scope`, can compile a list of script segments without needing to first concatenate them together into one large string.
