Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/compression.hh

@@ -0,0 +1,32 @@
+#pragma once
+///@file
+
+#include "ref.hh"
+#include "types.hh"
+#include "serialise.hh"
+
+#include <string>
+
+namespace nix {
+
+struct CompressionSink : BufferedSink, FinishSink
+{
+    using BufferedSink::operator();
+    using BufferedSink::writeUnbuffered;
+    using FinishSink::finish;
+};
+
+std::string decompress(const std::string & method, std::string_view in);
+
+std::unique_ptr<FinishSink> makeDecompressionSink(const std::string & method, Sink & nextSink);
+
+std::string compress(const std::string & method, std::string_view in, const bool parallel = false, int level = -1);
+
+ref<CompressionSink>
+makeCompressionSink(const std::string & method, Sink & nextSink, const bool parallel = false, int level = -1);
+
+MakeError(UnknownCompressionMethod, Error);
+
+MakeError(CompressionError, Error);
+
+}