Introduces a FastUnorderedMap class that uses FastVector. This type
uses static storage for up to a small fixed number of elements and
uses dynamic storage for anything larger.

Local testing shows this almost fully solves the regression from
using unordered_map. It's still slightly slower than using no
tracking (<5%).

Very degenerate cases which track dozens or more buffers will still
have significant overhead. For almost all applications that use only
a few buffers per RenderPass this will be very fast.

Test: angle_perftests, *vulkan_null_index_buffer_changed_ushort
Bug: angleproject:4950
Bug: angleproject:4429
Change-Id: I39edeaaa159124167f1ea23ad2e6eac5e9220d0b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2348108Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>