FTL: Add SmallMap<K, V, N>

SmallMap is a SmallVector of unordered key-value pairs. This CL includes
construction and lookup, while mutators will be added in a follow-up CL.

Bug: 160012986
Test: ftl_test
Change-Id: Id88485304c7d7f515226b4f8edf63ce9af6a2157