Expose the graphql operation as a constant in the generated genqlient file (#238)

This allows client code to see the operation (query or mutation) exactly
as genqlient sends it over the wire. This data was already available in
the generated safelist.json file, but now it's easily available from Go
code as well.
    
Fixes #236

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog

example/generated.go

@@ -69,6 +69,16 @@ func (v *getViewerViewerUser) GetMyName() string { return v.MyName }
 // GetCreatedAt returns getViewerViewerUser.CreatedAt, and is useful for accessing the field via an interface.
 func (v *getViewerViewerUser) GetCreatedAt() time.Time { return v.CreatedAt }
 
+// The query or mutation executed by getUser.
+const getUserOperation = `
+query getUser ($Login: String!) {
+	user(login: $Login) {
+		theirName: name
+		createdAt
+	}
+}
+`
+
 // getUser gets the given user's name from their username.
 func getUser(
 	ctx context.Context,
@@ -77,14 +87,7 @@ func getUser(
 ) (*getUserResponse, error) {
 	req := &graphql.Request{
 		OpName: "getUser",
-		Query: `
-query getUser ($Login: String!) {
-	user(login: $Login) {
-		theirName: name
-		createdAt
-	}
-}
-`,
+		Query:  getUserOperation,
 		Variables: &__getUserInput{
 			Login: Login,
 		},
@@ -103,20 +106,23 @@ query getUser ($Login: String!) {
 	return &data, err
 }
 
-func getViewer(
-	ctx context.Context,
-	client graphql.Client,
-) (*getViewerResponse, error) {
-	req := &graphql.Request{
-		OpName: "getViewer",
-		Query: `
+// The query or mutation executed by getViewer.
+const getViewerOperation = `
 query getViewer {
 	viewer {
 		MyName: name
 		createdAt
 	}
 }
-`,
+`
+
+func getViewer(
+	ctx context.Context,
+	client graphql.Client,
+) (*getViewerResponse, error) {
+	req := &graphql.Request{
+		OpName: "getViewer",
+		Query:  getViewerOperation,
 	}
 	var err error