Add release notes.

RELEASES.md

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+Rhai Release Notes
+==================
+
+Version 0.14.1
+==============
+
+The major features for this release is modules, script resource limits, and speed improvements
+(mainly due to avoiding allocations).
+
+New features
+------------
+
+* Modules and _module resolvers_ allow loading external scripts under a module namespace.
+  A module can contain constant variables, Rust functions and Rhai functions.
+* `export` variables and `private` functions.
+* _Indexers_ for Rust types.
+* Track script evaluation progress and terminate script run.
+* Set limit on maximum number of operations allowed per script run.
+* Set limit on maximum number of modules loaded per script run.
+* 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.
+* Stepped `range` function with a custom step.
+
+Speed improvements
+------------------
+
+### `StaticVec`
+
+A script contains many lists - statements in a block, arguments to a function call etc.
+In a typical script, most of these lists tend to be short - e.g. the vast majority of function calls contain
+fewer than 4 arguments, while most statement blocks have fewer than 4-5 statements, with one or two being
+the most common. Before, dynamic `Vec`'s are used to hold these short lists for very brief periods of time,
+causing allocations churn.
+
+In this version, large amounts of allocations are avoided by converting to a `StaticVec` -
+a list type based on a static array for a small number of items (currently four) -
+wherever possible plus other tricks. Most real-life scripts should see material speed increases.
+
+### Pre-computed variable lookups
+
+Almost all variable lookups, as well as lookups in loaded modules, are now pre-computed.
+A variable's name is almost never used to search for the variable in the current scope.
+
+_Getters_ and _setter_ function names are also pre-computed and cached, so no string allocations are
+performed during a property get/set call.
+
+### Pre-computed function call hashes
+
+Lookup of all function calls, including Rust and Rhai ones, are now through pre-computed hashes.
+The function name is no longer used to search for a function, making function call dispatches
+much faster.
+
+### Large Boxes for expressions and statements
+
+The expression (`Expr`) and statement (`Stmt`) types are modified so that all of the variants contain only
+one single `Box` to a large allocated structure containing _all_ the fields.  This makes the `Expr` and
+`Stmt` types very small (only one single pointer) and improves evaluation speed due to cache efficiency.
+
+Error handling
+--------------
+
+Previously, when an error occurs inside a function call, the error position reported is the function
+call site. This makes it difficult to diagnose the actual location of the error within the function.
+
+A new error variant `EvalAltResult::ErrorInFunctionCall` is added in this version.
+It wraps the internal error returned by the called function, including the error position within the function.