MMD leftover: logical_{or,and,xor}

[Leopold Toetsch]

The logical MMD functions currently try to set a passed in destination 
PMCs value via assign, or they return a clone of themselves.

This is verly likely wrong: not all PMCs have to implement assign:
<Dan>: That's why the destination controls assignment.
Additionaly we can't assign to singletons or r/o PMCs.
So the right thing for the logical functions seems to be, to just return 
the left or right side according to their boolean value (or return a new 
Boolean false in case of xor), i.e. you get a reference to one of the 
PMCs usually.

This would break a few tests, which don't cope with these ref semantics.
Comments welcome,
leo