feat(persistence): add snapshot save and load

lib/excache/persistence.ex

@@ -0,0 +1,22 @@
+defmodule Excache.Persistence do
+  @moduledoc false
+
+  def snapshot_path do
+    Application.get_env(:excache, :snapshot_path) ||
+      Path.join(to_string(:code.priv_dir(:excache)), "excache.snapshot")
+  end
+
+  def save do
+    path = snapshot_path()
+    File.mkdir_p!(Path.dirname(path))
+    File.write!(path, :erlang.term_to_binary(Excache.Store.dump()))
+    :ok
+  end
+
+  def load_entries(path) do
+    case File.read(path) do
+      {:ok, binary} -> :erlang.binary_to_term(binary)
+      {:error, _reason} -> []
+    end
+  end
+end