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/genqlient.yaml

@@ -76,11 +76,13 @@ client_getter: "github.com/you/yourpkg.GetClient"
 # A map from GraphQL type name to Go fully-qualified type name to override
 # the Go type genqlient will use for this GraphQL type.
 #
-# This is primarily used for custom scalars, or to map builtin scalars to
-# a nonstandard type.  By default, builtin scalars are mapped to the
-# obvious Go types (String and ID to string, Int to int, Float to float64,
-# and Boolean to bool), but this setting will extend or override those
-# mappings.
+# This is primarily used for custom scalars, or to map builtin scalars
+# to a nonstandard type that is defined elsewhere.  By default,
+# builtin scalars are mapped to the obvious Go types (String and ID to
+# string, Int to int, Float to float64, and Boolean to bool), but this
+# setting will extend or override those mappings.  (See also
+# @genqlient(typename: ...), which can be used to map builtin scalars
+# to a nonstandard type that genqlient defines for you.)
 #
 # genqlient does not validate these types in any way; they must define
 # whatever logic is needed (MarshalJSON/UnmarshalJSON or JSON tags) to