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/testdata/errors/BindingWithIncorrectSelection.go

@@ -0,0 +1,7 @@
+package errors
+
+const _ = `# @genqlient
+query GetPokemonWrongFields {
+  pokemon { species }
+}
+`

generate/testdata/errors/BindingWithIncorrectSelection.graphql

@@ -0,0 +1,3 @@
+query GetPokemonWrongFields {
+  pokemon { species species }
+}

generate/testdata/errors/BindingWithIncorrectSelection.schema.graphql

@@ -0,0 +1,8 @@
+type Query {
+  pokemon: Pokemon
+}
+
+type Pokemon {
+  species: String!
+  level: Int!
+}

generate/testdata/queries/Pokemon.graphql

@@ -8,7 +8,8 @@ query GetPokemonSiblings($input: PokemonInput!) {
     roles
     name
     # this is mapped globally to internal/testutil.Pokemon:
-    pokemon { species level }
+    # note field ordering matters, but whitespace shouldn't.
+    pokemon {    species level }
     # this overrides said mapping:
     # @genqlient(bind: "-")
     genqlientPokemon: pokemon { species level }

generate/testdata/snapshots/TestGenerateErrors-BindingWithIncorrectSelection.go

@@ -0,0 +1 @@
+invalid selection for type-binding GetPokemonWrongFieldsPokemon: testdata/errors/BindingWithIncorrectSelection.schema.graphql:2: expected 2 fields, got 1

generate/testdata/snapshots/TestGenerateErrors-BindingWithIncorrectSelection.graphql

@@ -0,0 +1 @@
+invalid selection for type-binding GetPokemonWrongFieldsPokemon: testdata/errors/BindingWithIncorrectSelection.graphql:2: expected field 1 to be level, got species