Standardize on _rungen.go for the file to contain go:generate commands.

Fixes issue #33.

datas/_rungen.go

@@ -0,0 +1,3 @@
+package datas
+
+//go:generate go run gen/types.go -o types.go

datas/datastore.go

@@ -7,8 +7,6 @@ import (
 	"github.com/attic-labs/noms/types"
 )
 
-//go:generate go run gen/types.go -o types.go
-
 type DataStore struct {
 	chunks.ChunkStore