Add Rhai book.

doc/src/engine/optimize/reoptimize.md

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+Re-Optimize an AST
+==================
+
+{{#include ../../links.md}}
+
+If it is ever needed to _re_-optimize an `AST`, use the `optimize_ast` method:
+
+```rust
+// Compile script to AST
+let ast = engine.compile("40 + 2")?;
+
+// Create a new 'Scope' - put constants in it to aid optimization if using 'OptimizationLevel::Full'
+let scope = Scope::new();
+
+// Re-optimize the AST
+let ast = engine.optimize_ast(&scope, &ast, OptimizationLevel::Full);
+```