Add support for inline fragments (#65)

## Summary:
In this commit I add support for inline fragments
(`... on MyType { fields }`) to genqlient.  This will make interfaces a
lot more useful!  In future commits I'll add named fragments, for which
we'll generate slightly different types, as discussed in DESIGN.md.

In general, implementing the flattening approach described in DESIGN.md
was... surprisingly easy.  All we have to do is recurse on applicable
fragments when generating our selection-set.  The refactor to
selection-set handling this encouraged was, I think, quite beneficial.
It did reveal two tricky pre-existing issues.

One issue is that GraphQL allows for duplicate selections, as long as
they match.  (In practice, this is only useful in the context of
fragments, although GraphQL allows it even without.) I decided to handle
the simple case (duplicate leaf fields; we just deduplicate) but leave
to the future the complex cases where we need to merge different
sub-selections (now #64).  For now we just forbid that; we can see how
much it comes up.

The other issue is that we are generating type-names incorrectly for
interface types; I had intended to do `MyInterfaceMyFieldMyType` for
shared fields and `MyImplMyFieldMyType` for non-shared ones, but instead
I did `MyFieldMyType`, which is inconsistent already and can result in
conflicts in the presence of fragments.  I'm going to fix this in a
separate commit, though, because it's going to require some refactoring
and is irrelevant to the main logic of this commit; I left some TODOs in
the tests related to this.

Issue: https://github.com/Khan/genqlient/issues/8

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, aberkan, MiguelCastillo

Required Reviewers: 

Approved by: dnerdy

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ✅ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/65

internal/integration/integration_test.go

@@ -206,6 +206,91 @@ func TestInterfaceListPointerField(t *testing.T) {
 	assert.Nil(t, *resp.Beings[2])
 }
 
+func TestFragments(t *testing.T) {
+	_ = `# @genqlient
+	query queryWithFragments($ids: [ID!]!) {
+		beings(ids: $ids) {
+			__typename id
+			... on Being { id name }
+			... on Animal {
+				id
+				hair { hasHair }
+				species
+				owner {
+					id
+					... on Being { name }
+					... on User { luckyNumber }
+				}
+			}
+			... on Lucky { luckyNumber }
+			... on User { hair { color } }
+		}
+	}`
+
+	ctx := context.Background()
+	server := server.RunServer()
+	defer server.Close()
+	client := graphql.NewClient(server.URL, http.DefaultClient)
+
+	resp, err := queryWithFragments(ctx, client, []string{"1", "3", "12847394823"})
+	require.NoError(t, err)
+
+	require.Len(t, resp.Beings, 3)
+
+	// We should get the following three beings:
+	//	User{Id: 1, Name: "Yours Truly"},
+	//	Animal{Id: 3, Name: "Fido"},
+	//	null
+
+	// Check fields both via interface and via type-assertion when possible
+	// User has, in total, the fields: __typename id name luckyNumber.
+	assert.Equal(t, "User", resp.Beings[0].GetTypename())
+	assert.Equal(t, "1", resp.Beings[0].GetId())
+	assert.Equal(t, "Yours Truly", resp.Beings[0].GetName())
+	// (hair and luckyNumber we need to cast for)
+
+	user, ok := resp.Beings[0].(*queryWithFragmentsBeingsUser)
+	require.Truef(t, ok, "got %T, not User", resp.Beings[0])
+	assert.Equal(t, "1", user.Id)
+	assert.Equal(t, "Yours Truly", user.Name)
+	// TODO(benkraft): Uncomment once we fix the interface-field type-naming
+	// bug that's causing this to get the wrong type (because we end up
+	// generating two conflicting types).
+	//	assert.Equal(t, "Black", user.Hair.Color)
+	assert.Equal(t, 17, user.LuckyNumber)
+
+	// Animal has, in total, the fields:
+	//	__typename
+	//	id
+	//	species
+	//	owner {
+	//		id
+	//		name
+	//		... on User { luckyNumber }
+	//	}
+	assert.Equal(t, "Animal", resp.Beings[1].GetTypename())
+	assert.Equal(t, "3", resp.Beings[1].GetId())
+	// (hair, species, and owner.* we have to cast for)
+
+	animal, ok := resp.Beings[1].(*queryWithFragmentsBeingsAnimal)
+	require.Truef(t, ok, "got %T, not Animal", resp.Beings[1])
+	assert.Equal(t, "3", animal.Id)
+	assert.Equal(t, SpeciesDog, animal.Species)
+	assert.True(t, animal.Hair.HasHair)
+
+	assert.Equal(t, "1", animal.Owner.GetId())
+	assert.Equal(t, "Yours Truly", animal.Owner.GetName())
+	// (luckyNumber we have to cast for, again)
+
+	owner, ok := animal.Owner.(*queryWithFragmentsBeingsOwnerUser)
+	require.Truef(t, ok, "got %T, not User", animal.Owner)
+	assert.Equal(t, "1", owner.Id)
+	assert.Equal(t, "Yours Truly", owner.Name)
+	assert.Equal(t, 17, owner.LuckyNumber)
+
+	assert.Nil(t, resp.Beings[2])
+}
+
 func TestGeneratedCode(t *testing.T) {
 	// TODO(benkraft): Check that gqlgen is up to date too.  In practice that's
 	// less likely to be a problem, since it should only change if you update