Implement type union/intersection for nested references

[shans]

(from b/156983624)

schema intersection has been implemented so that reference fields will be dropped unless their contained types match exactly.

Probably this should be implemented by instead performing an intersection of their contained types.

e.g. Foo { ref: &Person { name, age } } intersect Foo { ref: &Person { name, height } } should result in Foo { ref: &Person { name } }

TS impl at https://github.com/PolymerLabs/arcs/blob/master/src/runtime/schema.ts#L158