LibJS: Keep parsed function parameters in a shared data structure

Instead of making a copy of the Vector<FunctionParameter> from the AST
every time we instantiate an ECMAScriptFunctionObject, we now keep the
parameters in a ref-counted FunctionParameters object.

This reduces memory usage, and also allows us to cache the bytecode
executables for default parameter expressions without recompiling them
for every instantiation. :^)

Libraries/LibJS/Forward.h

@@ -179,6 +179,7 @@ class ForStatement;
 class FunctionEnvironment;
 class FunctionNode;
 struct FunctionParameter;
+class FunctionParameters;
 class GlobalEnvironment;
 class GlobalObject;
 struct GraphLoadingState;