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/convert.go

@@ -289,7 +289,7 @@ func (g *generator) convertDefinition(
 		}, err
 	}
 	goBuiltinName, ok := builtinTypes[def.Name]
-	if ok {
+	if ok && options.TypeName == "" {
 		return &goOpaqueType{GoRef: goBuiltinName, GraphQLName: def.Name}, nil
 	}
 
@@ -479,6 +479,17 @@ func (g *generator) convertDefinition(
 		return g.addType(goType, goType.GoName, pos)
 
 	case ast.Scalar:
+		if builtinTypes[def.Name] != "" {
+			// In this case, the user asked for a custom Go type-name
+			// for a built-in type, e.g. `type MyString string`.
+			goType := &goTypenameForBuiltinType{
+				GoTypeName:    name,
+				GoBuiltinName: builtinTypes[def.Name],
+				GraphQLName:   def.Name,
+			}
+			return g.addType(goType, goType.GoTypeName, pos)
+		}
+
 		// (If you had an entry in bindings, we would have returned it above.)
 		return nil, errorf(
 			pos, `unknown scalar %v: please add it to "bindings" in genqlient.yaml`, def.Name)