Add FileWriter class to aos/util

This allows for a malloc-free writer for convenience.

Change-Id: I80cf6ac68f9190106fcb57d85cf758a5d491f04b
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/util/BUILD b/aos/util/BUILD
index bd1dbab..3731965 100644
--- a/aos/util/BUILD
+++ b/aos/util/BUILD
@@ -336,6 +336,7 @@
         ":file",
         "//aos:realtime",
         "//aos/testing:googletest",
+        "//aos/testing:tmpdir",
     ],
 )