Add a flag to disable array covariance to experiment with for lw2

Design discussions wish to look at restricting array covariance.

LW1 rules:

  * [QFoo; <: [LFoo; <: [Ljava/lang/Object;

LW2 proposed rules:

  * [LFoo; <: [Ljava/lang/Object;
  * [QFoo; <: Ljava/lang/Object; (i.e. no array covariance for unboxed value arrays
     ** however System.arraycopy accepts both [LFoo; and [QFoo; as source and destination (i.e. mix array of "boxed" values and array "unboxed" values)