fuchsia: FIDL JS: support nested structs in responses

Implements vector and string decoding for responses and fixes/tests
struct-in-struct.

Bug: 883496
Change-Id: I9d518103fb28b93802a487765f5fefb65ef01ea1
Reviewed-on: https://chromium-review.googlesource.com/c/1246958
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600537}
5 files changed