LibJS: Add NativeFunction, a callable wrapper around a C++ lambda

This can be used to implement arbitrary functionality, callable from
JavaScript.

To make this work, I had to change the way CallExpression passes
arguments to the callee. Instead of a HashMap<String, Value>, we now
pass an ordered list of Argument { String name; Value value; }.

This patch includes a native "print(argument)" function. :^)

Libraries/LibJS/Forward.h

@@ -29,6 +29,7 @@
 namespace JS {
 
 class ASTNode;
+class Argument;
 class Cell;
 class Expression;
 class Heap;