Allow creating aliases for builtin types, using typename. (#133)

## Summary:
This lets you write code like:
```
query x {
   # @genqlient(typename: "MyString")
   someStringField
}
```
and genqlient will do
```
typename MyString string
type x struct {
   someStringField MyString
}
```

This was not difficult to implement, though it required introducing a
new identifier type.  The main difficulty I had was weird test
failures, that it turns out was due to the tests putting a bunch of
fields on the same line, so that the genqlient directive on the
previous line applied to all of them, accidentally.  This became a
problem when `typename` suddenly started being respected for builtin
types!  I fixed it by just spreading out the queries a bit.

Fixes #130

## Test plan:
make check

Author: csilvers

Reviewers: dnerdy, StevenACoffman, benjaminjkraft

Required Reviewers:

Approved By: StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/133

generate/testdata/errors/ConflictingTypeNames.graphql

@@ -1,6 +1,11 @@
 query ConflictingTypeNames {
   # @genqlient(typename: "T")
-  f { g }
+  f {
+    g
+  }
   # @genqlient(typename: "T")
-  otherF: f { g h }
+  otherF: f {
+    g
+    h
+  }
 }

generate/testdata/queries/TypeNames.graphql

@@ -1,10 +1,20 @@
 # @genqlient(typename: "Resp")
 query TypeNames {
   # @genqlient(typename: "User")
-  user { id name }
+  user {
+    id
+    name
+  }
   # @genqlient(typename: "Item")
-  randomItem { id name }
+  randomItem {
+     id
+     # @genqlient(typename: "NameType")
+     name
+  }
   # (ok to reuse the name as long as they match)
   # @genqlient(typename: "User")
-  users { id name }
+  users {
+    id
+    name
+  }
 }

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

@@ -29,7 +29,7 @@ type Item interface {
 	// ID is the identifier of the content.
 	GetId() testutil.ID
 	// GetName returns the interface-field "name" from its implementation.
-	GetName() string
+	GetName() NameType
 }
 
 func (v *ItemArticle) implementsGraphQLInterfaceItem() {}
@@ -109,7 +109,7 @@ type ItemArticle struct {
 	Typename string `json:"__typename"`
 	// ID is the identifier of the content.
 	Id   testutil.ID `json:"id"`
-	Name string      `json:"name"`
+	Name NameType    `json:"name"`
 }
 
 // GetTypename returns ItemArticle.Typename, and is useful for accessing the field via an interface.
@@ -119,14 +119,14 @@ func (v *ItemArticle) GetTypename() string { return v.Typename }
 func (v *ItemArticle) GetId() testutil.ID { return v.Id }
 
 // GetName returns ItemArticle.Name, and is useful for accessing the field via an interface.
-func (v *ItemArticle) GetName() string { return v.Name }
+func (v *ItemArticle) GetName() NameType { return v.Name }
 
 // ItemTopic includes the requested fields of the GraphQL type Topic.
 type ItemTopic struct {
 	Typename string `json:"__typename"`
 	// ID is the identifier of the content.
 	Id   testutil.ID `json:"id"`
-	Name string      `json:"name"`
+	Name NameType    `json:"name"`
 }
 
 // GetTypename returns ItemTopic.Typename, and is useful for accessing the field via an interface.
@@ -136,14 +136,14 @@ func (v *ItemTopic) GetTypename() string { return v.Typename }
 func (v *ItemTopic) GetId() testutil.ID { return v.Id }
 
 // GetName returns ItemTopic.Name, and is useful for accessing the field via an interface.
-func (v *ItemTopic) GetName() string { return v.Name }
+func (v *ItemTopic) GetName() NameType { return v.Name }
 
 // ItemVideo includes the requested fields of the GraphQL type Video.
 type ItemVideo struct {
 	Typename string `json:"__typename"`
 	// ID is the identifier of the content.
 	Id   testutil.ID `json:"id"`
-	Name string      `json:"name"`
+	Name NameType    `json:"name"`
 }
 
 // GetTypename returns ItemVideo.Typename, and is useful for accessing the field via an interface.
@@ -153,7 +153,9 @@ func (v *ItemVideo) GetTypename() string { return v.Typename }
 func (v *ItemVideo) GetId() testutil.ID { return v.Id }
 
 // GetName returns ItemVideo.Name, and is useful for accessing the field via an interface.
-func (v *ItemVideo) GetName() string { return v.Name }
+func (v *ItemVideo) GetName() NameType { return v.Name }
+
+type NameType string
 
 // Resp is returned by TypeNames on success.
 type Resp struct {