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

docs/CHANGELOG.md

@@ -26,6 +26,8 @@ When releasing a new version:
 
 - genqlient now generates getter methods for all fields, even those which do not implement a genqlient-generated interface; this can be useful for callers who wish to define their own interface and have several unrelated genqlient types which have the same fields implement it.
 - genqlient config now accepts either a single or multiple schema files for the `schema` field.
+- The `typename` option can now be used on basic types (string, int, etc) as well as structs; this can be useful to have genqlient define new types like `type Language string` and use that type for specified fields.
+
 ### Bug fixes:
 
 - In certain very rare cases involving duplicate fields in fragment spreads, genqlient would generate code that failed to compile due to duplicate methods not getting promoted; genqlient now generates correct types.  (See #126 for a more complete description.)