Support dictionary union

[withered-magic]

From the starlark spec

The binary | operation may be applied to two dictionaries. It yields a new dictionary whose set of keys is the union of the sets of keys of the two operands. The corresponding values are taken from the operands, where the value taken from the right operand takes precedence if both contain a given key. Iterating over the keys in the resulting dictionary first yields all keys in the left operand in insertion order, then all keys in the right operand that were not present in the left operand, again in insertion order.