dfa: Experimental implementation of three possible optimizations

[fridis]

• For certain clazzes, do not create ValueSet but, in case of a set, use one global set for all sets.

• For cetain clazzes, do not create separate Instances depending on context, but create only one single instance

• Do not create ValueSets of RefValue of the same clazz, but instead create RefValue of ValueSet, i.e., instead of

    {boxed(v1), boxed(v2)}

create

    boxed({v1,v2})

All these can be enabled by static fields in DFA

  static boolean ONLY_ONE_VALUE_SET = false; 
  static boolean ONLY_ONE_INSTANCE  = false;
  static boolean NO_SET_OF_REFS     = false;