Pass field names more accessibly to object resolvers

Currently, if you want to look at the set of fields being requested by a child object in a resolver, you have to look deep in the AST, with an expression like this:

```
(field.name.value for field in info.field_asts[0].selection_set.selections)
```

It would be nice if these names were attached directly to the `info` object, even as a property, so that things like database column selection are easier to implement.