Move /src to /subprojects

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

subprojects/libutil/xml-writer.hh

@@ -0,0 +1,66 @@
+#pragma once
+///@file
+
+#include <iostream>
+#include <string>
+#include <list>
+#include <map>
+
+
+namespace nix {
+
+
+typedef std::map<std::string, std::string> XMLAttrs;
+
+
+class XMLWriter
+{
+private:
+
+    std::ostream & output;
+
+    bool indent;
+    bool closed;
+
+    std::list<std::string> pendingElems;
+
+public:
+
+    XMLWriter(bool indent, std::ostream & output);
+    ~XMLWriter();
+
+    void close();
+
+    void openElement(std::string_view name,
+        const XMLAttrs & attrs = XMLAttrs());
+    void closeElement();
+
+    void writeEmptyElement(std::string_view name,
+        const XMLAttrs & attrs = XMLAttrs());
+
+private:
+    void writeAttrs(const XMLAttrs & attrs);
+
+    void indent_(size_t depth);
+};
+
+
+class XMLOpenElement
+{
+private:
+    XMLWriter & writer;
+public:
+    XMLOpenElement(XMLWriter & writer, std::string_view name,
+        const XMLAttrs & attrs = XMLAttrs())
+        : writer(writer)
+    {
+        writer.openElement(name, attrs);
+    }
+    ~XMLOpenElement()
+    {
+        writer.closeElement();
+    }
+};
+
+
+}