Instead of every NativeFunction callback having to ask the Interpreter
for the current "this" value and then converting it to an Object etc,
just pass "this" as an Object* directly.
Both types of functions are now Function and implement calling via:
virtual Value call(Interpreter&, Vector<Value> arguments);
This removes the need for CallExpression::execute() to care about which
kind of function it's calling. :^)
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. :^)
