`OverlappingFieldsCanBeMerged` would infinite loop when passed something like

```graphql
fragment A on User {
  name
  ...A
}
```

It's not `OverlappingFieldsCanBeMerged`'s responsibility to detect that validation error, but we still would ideally avoid infinite looping.

This detects that case, and pretends that the infinite spread wasn't there for the purposes of this validation step.

Also, by memoizing and checking for self-references this removes duplicate reports.

Closes #780