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/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
+  }
 }