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.
This patch adds NativeProperty, which can be used to implement object
properties that have C++ getters and/or setters.
Use this to move String.prototype.length to its correct place. :^)
This patch adds String.prototype.charAt() to demonstrate that prototype
property lookup works, and that you can call a prototype function on an
object, and it will do what you expect. :^)
