Problem: Oberon Compiler destroys procedure stack image when assigning a string to an ARRAY OF CHAR variable.

[sam-falvo]

Originally from KestrelComputer/kestrel#223

Two solutions exist:

1. IF destination is an open array, THEN issue a compiler error, OR,
2. IF destination is an open array AND source is a constant string item, THEN just set the pointer and length attributes of the open array.

To reproduce, see https://gist.github.com/sam-falvo/7a138b708bebc5c292d6