Codegen for enum types

This generates a flow type for enums:

```noms
enum Handedness {
  right
  left
  switch
}
```

Generates

```js
type E =
  0 |  // right
  1 |  // left
  2;  // switch
```

Issue #1081

nomdl/codegen/code/generate.go

@@ -10,6 +10,7 @@ package code
 
 import (
 	"fmt"
+	"reflect"
 	"strings"
 	"unicode"
 
@@ -447,6 +448,11 @@ func (gen Generator) ToTypeJS(t types.Type, fileID, nomsName string, indent int)
 	panic("Unreachable")
 }
 
+// IsLast determines if |index| is the last index in |seq|.
+func (gen Generator) IsLast(index int, seq interface{}) bool {
+	return reflect.ValueOf(seq).Len() == index+1
+}
+
 // ToTag replaces "-" characters in s with "_", so it can be used in a Go identifier.
 // TODO: replace other illegal chars as well?
 func ToTag(r ref.Ref) string {

nomdl/codegen/codegen.go

@@ -280,6 +280,7 @@ func (gen *codeGen) readTemplates() *template.Template {
 			"valueToDef":    gen.generator.ValueToDef,
 			"valueToUser":   gen.generator.ValueToUser,
 			"valueZero":     gen.generator.ValueZero,
+			"isLast":        gen.generator.IsLast,
 		}).ParseGlob(glob))
 }
 

nomdl/codegen/js/enum.tmpl

@@ -1 +1,3 @@
-// enum.tmpl
+{{$name := .Name}}
+export type {{.Name}} ={{range $index, $id := .Ids}}
+  {{$index}}{{if isLast $index $.Ids | not}} |{{else}};{{end}}  // {{$id}}{{end}}

nomdl/codegen/test/gen/enum_struct.noms.js

@@ -20,7 +20,11 @@ import * as _noms from '@attic/noms';
   _noms.registerPackage(pkg);
 }
 
-// enum.tmpl
+
+export type Handedness =
+  0 |  // right
+  1 |  // left
+  2;  // switch
 
 export interface EnumStruct extends _noms.Struct {
   hand: Handedness;  // readonly

nomdl/codegen/test/gen/sha1_1c216c6.js

@@ -28,4 +28,8 @@ export interface S extends _noms.Struct {
   b: boolean;  // readonly
   setB(value: boolean): S;
 }
-// enum.tmpl
+
+export type E =
+  0 |  // e1
+  1 |  // e2
+  2;  // e3

nomdl/codegen/test/gen/struct_with_imports.noms.js

@@ -22,7 +22,10 @@ import * as _noms from '@attic/noms';
   _noms.registerPackage(pkg);
 }
 
-// enum.tmpl
+
+export type LocalE =
+  0 |  // LocalE1
+  1;  // Ignored
 
 export interface ImportUser extends _noms.Struct {
   importedStruct: D;  // readonly

nomdl/codegen/testDeps/leafDep/leafDep.noms.js

@@ -28,4 +28,8 @@ export interface S extends _noms.Struct {
   b: boolean;  // readonly
   setB(value: boolean): S;
 }
-// enum.tmpl
+
+export type E =
+  0 |  // e1
+  1 |  // e2
+  2;  // e3