If requested, validate binding-types get the right fields (#70)

## Summary:
One sharp edge of the new `bindings` setting (when used for composite
types) is this: the (presumably struct) type to which you're binding
may expect to have particular fields, but it's GraphQL so you could have
requested some other set of fields.  Now, if you ask us, we check.

Specifically, I've added a new setting under the `bindings` items, which
says: everywhere we query this must select these fields.  (Or use its
own inline `# @genqlient(bind: ...)`.)  It must select exactly those
fields, in order, no more, no less.  This was fairly easy to implement;
actually comparing the selections was surprisingly much code but it's
all pretty straightforward.

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, aberkan, csilvers, 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/70

generate/generate_test.go

@@ -52,11 +52,14 @@ func TestGenerate(t *testing.T) {
 				Generated:        goFilename,
 				ExportOperations: queriesFilename,
 				Bindings: map[string]*TypeBinding{
-					"ID":           {Type: "github.com/Khan/genqlient/internal/testutil.ID"},
-					"DateTime":     {Type: "time.Time"},
-					"Junk":         {Type: "interface{}"},
-					"ComplexJunk":  {Type: "[]map[string]*[]*map[string]interface{}"},
-					"Pokemon":      {Type: "github.com/Khan/genqlient/internal/testutil.Pokemon"},
+					"ID":          {Type: "github.com/Khan/genqlient/internal/testutil.ID"},
+					"DateTime":    {Type: "time.Time"},
+					"Junk":        {Type: "interface{}"},
+					"ComplexJunk": {Type: "[]map[string]*[]*map[string]interface{}"},
+					"Pokemon": {
+						Type:              "github.com/Khan/genqlient/internal/testutil.Pokemon",
+						ExpectExactFields: "{ species level }",
+					},
 					"PokemonInput": {Type: "github.com/Khan/genqlient/internal/testutil.Pokemon"},
 				},
 				AllowBrokenFeatures: true,
@@ -117,6 +120,14 @@ func TestGenerate(t *testing.T) {
 	}
 }
 
+// TestGenerate is a snapshot-based test of error text.
+//
+// For each .go or .graphql file in testdata/errors, and corresponding
+// .schema.graphql file, it asserts that the given query returns an error, and
+// that that error's string-text matches the snapshot.  The snapshotting is
+// useful to ensure we don't accidentally make the text less readable, drop the
+// line numbers, etc.  We include both .go and .graphql tests, to make sure the
+// line numbers work in both cases.
 func TestGenerateErrors(t *testing.T) {
 	files, err := ioutil.ReadDir(errorsDir)
 	if err != nil {
@@ -143,6 +154,10 @@ func TestGenerateErrors(t *testing.T) {
 				Bindings: map[string]*TypeBinding{
 					"ValidScalar":   {Type: "string"},
 					"InvalidScalar": {Type: "bogus"},
+					"Pokemon": {
+						Type:              "github.com/Khan/genqlient/internal/testutil.Pokemon",
+						ExpectExactFields: "{ species level }",
+					},
 				},
 				AllowBrokenFeatures: true,
 			})