Add support for interfaces, part 3: automatically add __typename (#56)

## Summary:
In #52, I added support for interface types, but with the simplifying
restriction (among others) that the user must request the field
`__typename`.  In this commit, I remove this restriction.

The basic idea is simple: we preprocess the query to add `__typename`.
The implementation isn't much more complicated!  Although it required
some new wiring in a few places.

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/56

generate/testdata/snapshots/TestGenerate-InterfaceNesting.graphql-InterfaceNesting.graphql.go

@@ -65,7 +65,6 @@ type InterfaceNestingRootTopicChildrenArticle struct {
 
 // InterfaceNestingRootTopicChildrenArticleParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenArticleParentTopic struct {
-	Typename string `json:"__typename"`
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                          `json:"id"`
 	Children []InterfaceNestingRootTopicChildrenArticleParentTopicChildrenContent `json:"-"`
@@ -223,7 +222,6 @@ type InterfaceNestingRootTopicChildrenTopic struct {
 
 // InterfaceNestingRootTopicChildrenTopicParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenTopicParentTopic struct {
-	Typename string `json:"__typename"`
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                        `json:"id"`
 	Children []InterfaceNestingRootTopicChildrenTopicParentTopicChildrenContent `json:"-"`
@@ -337,7 +335,6 @@ type InterfaceNestingRootTopicChildrenVideo struct {
 
 // InterfaceNestingRootTopicChildrenVideoParentTopic includes the requested fields of the GraphQL type Topic.
 type InterfaceNestingRootTopicChildrenVideoParentTopic struct {
-	Typename string `json:"__typename"`
 	// ID is documented in the Content interface.
 	Id       testutil.ID                                                        `json:"id"`
 	Children []InterfaceNestingRootTopicChildrenVideoParentTopicChildrenContent `json:"-"`
@@ -456,7 +453,6 @@ query InterfaceNesting {
 			__typename
 			id
 			parent {
-				__typename
 				id
 				children {
 					__typename