Added slot to hold imported closed-over variables.
Added instruction for fetching closed-over variable cell.
Export name of accessor of recently-added slot.
Export name of new FETCH instruction and of its constructor.
Added label for drawing new FETCH instruction.
Added new instruction for reading a cell of a closed-over variable.
Export name of new READ-CELL instruction and of its constructor.
Added label for drawing new READ-CELL instruction.
Added new instruction for writing a cell of a closed-over variable.
Export name of new WRITE-CELL instruction and of its constructor.
Added label for drawing new WRITE-CELL instruction.