Add support for "flattening" fragment-spreads (#121)

## Summary:
One common use of fragment spreads is as the entirety of a field's
selection, e.g.
```graphql
query MyQuery {
  myField {
    ...MyFragment
  }
}
```
In this case, by default, genqlient generates a wrapper type
`MyQueryMyFieldMyType`, which just embeds `MyFragment`.  This makes
sense if you later want to add more fields in addition to the fragment
spread.  But if you don't -- and you did the fragment because you want
to share types, it's an extra layer of indirection.  (Which becomes
especially onerous if `myField` has list type (`[MyType!]`), such that
it's not just an extra attribute-access to get to `MyFragment`.)

The new option `# @genqlient(flatten: true)` simplifies this situation:
if applied to `myField` is skips the wrapper type;
`MyQueryResponse.MyField` will simply have type `MyFragment` (or
`[]MyFragment`, or whatever).  This should hopefully make the `typename`
option, which has more limitations, less necessary.

Note that in #30 the initial idea was to support this for fields as
well.  This would require significant additional complexity in the
JSON-(un)marshaling code, and has proven less necessary, so I
implemented this option only for fragment-spreads for now.  With that
restriction, it was shockingly simple; we have to hook into a bunch of
different places, but they're all quite simple, since the structure of
the Go types still matches the structure in GraphQL.

Issue: https://github.com/Khan/genqlient/issues/30

## Test plan:
make check


Author: benjaminjkraft

Reviewers: csilvers, dnerdy, aberkan, jvoll, mahtabsabet, MiguelCastillo, StevenACoffman

Required Reviewers: 

Approved By: csilvers, dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/121

generate/testdata/queries/Flatten.graphql

@@ -0,0 +1,42 @@
+# @genqlient(flatten: true)
+fragment QueryFragment on Query {
+  ...InnerQueryFragment
+}
+
+fragment InnerQueryFragment on Query {
+  # @genqlient(flatten: true)
+  randomVideo {
+    ...VideoFields
+  }
+  # @genqlient(flatten: true)
+  randomItem {
+    ...ContentFields
+  }
+  # @genqlient(flatten: true)
+  otherVideo: randomVideo {
+    ...ContentFields
+  }
+}
+
+fragment VideoFields on Video {
+  id
+  parent {
+    # @genqlient(flatten: true)
+    videoChildren {
+      ...ChildVideoFields
+    }
+  }
+}
+
+fragment ChildVideoFields on Video {
+  id name
+}
+
+fragment ContentFields on Content {
+  name url
+}
+
+# @genqlient(flatten: true)
+query ComplexNamedFragments {
+  ...QueryFragment
+}

generate/testdata/queries/schema.graphql

@@ -131,6 +131,7 @@ type Topic implements Content {
   parent: Topic
   url: String!
   children: [Content!]!
+  videoChildren: [Video!]!
   schoolGrade: String
 }
 
@@ -162,6 +163,7 @@ type Query {
   root: Topic!
   randomItem: Content!
   randomLeaf: LeafContent!
+  randomVideo: Video!
   convert(dt: DateTime!, tz: String): DateTime!
   maybeConvert(dt: DateTime, tz: String): DateTime
   getJunk: Junk