Add O(1)-memory copyPath() function

src/libutil/archive.cc

@@ -375,4 +375,13 @@ void copyNAR(Source & source, Sink & sink)
 }
 
 
+void copyPath(const Path & from, const Path & to)
+{
+    auto source = sinkToSource([&](Sink & sink) {
+        dumpPath(from, sink);
+    });
+    restorePath(to, *source);
+}
+
+
 }

src/libutil/archive.hh

@@ -77,6 +77,8 @@ void restorePath(const Path & path, Source & source);
 /* Read a NAR from 'source' and write it to 'sink'. */
 void copyNAR(Source & source, Sink & sink);
 
+void copyPath(const Path & from, const Path & to);
+
 
 extern const std::string narVersionMagic1;