Add a += and + operator for dictionaries

[dcbaker]

Like for arrays, I'd imagine that if you wrote something like:

x = {'foo' : 1}
x += {'bar': 2}

that this would be equivalent to:

x = {'foo' : 1} + {'bar': 2}

The only question that remains is what to do with key collisions. There are two possibilities that seem acceptable to me:

1. The RHS dict overrides the LHS
   Equivalent python

   d = d1.copy()
   d.update(d2)
   return d

2. keys existing in both dicts is an error