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. :^)
To make sure that everyone has the same instance of StringPrototype,
hang a global prototype off of the Interpreter that can be fetched
when constructing new StringObjects.
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. :^)
