feat(app): add otp application and public api

lib/excache.ex

@@ -0,0 +1,9 @@
+defmodule Excache do
+  @moduledoc """
+  In-memory key-value server speaking a subset of the RESP2 protocol.
+  """
+
+  defdelegate save(), to: Excache.Persistence
+  defdelegate flushdb(), to: Excache.Store
+  defdelegate dbsize(), to: Excache.Store
+end