Add Rhai book.

doc/src/engine/optimize/eager.md

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+Eager Function Evaluation When Using Full Optimization Level
+==========================================================
+
+{{#include ../../links.md}}
+
+When the optimization level is [`OptimizationLevel::Full`], the [`Engine`] assumes all functions to be _pure_ and will _eagerly_
+evaluated all function calls with constant arguments, using the result to replace the call.
+
+This also applies to all operators (which are implemented as functions).
+
+For instance, the same example above:
+
+```rust
+// When compiling the following with OptimizationLevel::Full...
+
+const DECISION = 1;
+                            // this condition is now eliminated because 'DECISION == 1'
+if DECISION == 1 {          // is a function call to the '==' function, and it returns 'true'
+    print("hello!");        // this block is promoted to the parent level
+} else {
+    print("boo!");          // this block is eliminated because it is never reached
+}
+
+print("hello!");            // <- the above is equivalent to this
+                            //    ('print' and 'debug' are handled specially)
+```
+
+Because of the eager evaluation of functions, many constant expressions will be evaluated and replaced by the result.
+This does not happen with [`OptimizationLevel::Simple`] which doesn't assume all functions to be _pure_.
+
+```rust
+// When compiling the following with OptimizationLevel::Full...
+
+let x = (1+2)*3-4/5%6;      // <- will be replaced by 'let x = 9'
+let y = (1>2) || (3<=4);    // <- will be replaced by 'let y = true'
+```