Move /src to /subprojects

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

subprojects/libutil/hilite.hh

@@ -0,0 +1,23 @@
+#pragma once
+///@file
+
+#include <regex>
+#include <vector>
+#include <string>
+
+namespace nix {
+
+/**
+ * Highlight all the given matches in the given string `s` by wrapping
+ * them between `prefix` and `postfix`.
+ *
+ * If some matches overlap, then their union will be wrapped rather
+ * than the individual matches.
+ */
+std::string hiliteMatches(
+    std::string_view s,
+    std::vector<std::smatch> matches,
+    std::string_view prefix,
+    std::string_view postfix);
+
+}