Fuchsia: Implement vector<string>, fix traversal order

Previously the body of structs was traversed in-order, but out-of-line
items were deferred to a queue, making traversal mostly breadth-first.
Instead, fix encoding by emitting any out-of-line items by allocating
(and filling) directly following the current item.

Bug: 883496
Change-Id: If157930521e4cf5f89f702207defe91beb862def
Reviewed-on: https://chromium-review.googlesource.com/c/1316429
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605745}
4 files changed