Added basic CDR opcode implementation
Added basic CDR opcode implementation

Currently doesn't check if the source register forms a list or not. Also
might not handle references to other cells properly. If the cdr of a
cell points to another cell, the destination register should probably
dereference cell being pointed to, and set the car and cdr of the
destination register to the car and cdr of the cell being pointed to.
This way the destination cell can be read identcially to what the source
register's cdr was pointing to, if it was pointing to another cell.

donnyward/portablescheme,donnyward/portablescheme,donnyward/portablescheme