Rename FastUnordered types to FlatUnordered. These names are consistent with the common parlance. Flat indicates the values are packed in memory, while fast indicates that lookup is as fast as possible. Bug: angleproject:6776 Change-Id: I7b56af26d7fdbf5956872be5033c3aa3f6b1b8c7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3484978 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org> Auto-Submit: Jamie Madill <jmadill@chromium.org>