Removes IAS and IAG instructions and adds a new register access
code to the specification's operand listing. 
All unit tests have been fixed to reflect these new values.

The IA register had its own get/set instructions, because it was added
to the DCPU spec later on and it seems Notch was not willing to change
the rest of the spec to fit it in properly. Luckily, we do not have to worry
about such things at this stage of development.