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

internal/integration/integration_test.go

@@ -59,7 +59,7 @@ func TestVariables(t *testing.T) {
 func TestInterfaceNoFragments(t *testing.T) {
 	_ = `# @genqlient
 	query queryWithInterfaceNoFragments($id: ID!) {
-		being(id: $id) { __typename id name }
+		being(id: $id) { id name }
 		me { id name }
 	}`
 
@@ -102,7 +102,7 @@ func TestInterfaceNoFragments(t *testing.T) {
 func TestInterfaceListField(t *testing.T) {
 	_ = `# @genqlient
 	query queryWithInterfaceListField($ids: [ID!]!) {
-		beings(ids: $ids) { __typename id name }
+		beings(ids: $ids) { id name }
 	}`
 
 	ctx := context.Background()