Persistently cache InputAccessor::fetchToStore()

This avoids repeated copying of the same source tree between Nix
invocations. It requires the accessor to have a "fingerprint" (e.g. a
Git revision) that uniquely determines its contents.

src/libstore/content-address.hh

@@ -39,12 +39,12 @@ enum struct FileIngestionMethod : uint8_t {
     /**
      * Flat-file hashing. Directly ingest the contents of a single file
      */
-    Flat = false,
+    Flat = 0,
     /**
      * Recursive (or NAR) hashing. Serializes the file-system object in Nix
      * Archive format and ingest that
      */
-    Recursive = true
+    Recursive = 1
 };
 
 /**