BinaryCacheStore::readFile(): Return a shared_ptr to a string

This allows readFile() to indicate that a file doesn't exist, and
might eliminate some large string copying.

src/libutil/compression.hh

@@ -1,11 +1,13 @@
 #pragma once
 
+#include "ref.hh"
+
 #include <string>
 
 namespace nix {
 
 std::string compressXZ(const std::string & in);
 
-std::string decompressXZ(const std::string & in);
+ref<std::string> decompressXZ(const std::string & in);
 
 }