add support for defining a [ ] method for getting/setting values

Currently getting/setting values from hashes and arrays is done through a secret internal interface. Remove this, and instead allow '[]' to be defined in it's own right as a method, like in Ruby.

The 'get' and 'set' methods on QubyArray and QubyHash will need to be moved out to the standard library.

Then add support for using this in strings.
