FTL: Add SmallMap mutators

Add try_emplace, try_replace, emplace_or_replace, and erase. Rename
getters so that `find` returns an iterator like standard containers.

Bug: 185536303
Test: ftl_test
Change-Id: I095bb800c6bd786b9fceaf632aa06b2d6cdadb71
diff --git a/include/ftl/small_vector.h b/include/ftl/small_vector.h
index cb0ae35..0341435 100644
--- a/include/ftl/small_vector.h
+++ b/include/ftl/small_vector.h
@@ -348,7 +348,7 @@
   using Impl::pop_back;
 
   void unstable_erase(iterator it) {
-    if (it != last()) std::iter_swap(it, last());
+    if (it != last()) replace(it, std::move(back()));
     pop_back();
   }