Correctly handle self-recursive types automatically

(Split from #149.)

If you have in your schema
```graphql
input T {
f: T # or `f: T!`
}
```
then genqlient by default generates
```go
struct T {
F T
}
```
which does not compile. Of course you can manually add `pointer: true`, but we could just do that automatically because it's the only valid way to represent this. Apparently this comes up a lot with Hasura-generated schemas, so although it seems a bit special-casey it's probably worth handling.

Start at the generator path that turns GraphQL input types into Go structs and inspect how the existing `pointer: true` option is applied. Add coverage for `input T { f: T }` and `f: T!`, then verify the generated Go compiles without manual configuration.

Written by the indexing model from the issue text.