Move /src to /subprojects

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

subprojects/libutil/variant-wrapper.hh

@@ -0,0 +1,30 @@
+#pragma once
+///@file
+
+// not used, but will be used by callers
+#include <variant>
+
+/**
+ * Force the default versions of all constructors (copy, move, copy
+ * assignment).
+ */
+#define FORCE_DEFAULT_CONSTRUCTORS(CLASS_NAME) \
+    CLASS_NAME(const CLASS_NAME &) = default; \
+    CLASS_NAME(CLASS_NAME &) = default; \
+    CLASS_NAME(CLASS_NAME &&) = default; \
+    \
+    CLASS_NAME & operator =(const CLASS_NAME &) = default; \
+    CLASS_NAME & operator =(CLASS_NAME &) = default;
+
+/**
+ * Make a wrapper constructor. All args are forwarded to the
+ * construction of the "raw" field. (Which we assume is the only one.)
+ *
+ * The moral equivalent of `using Raw::Raw;`
+ */
+#define MAKE_WRAPPER_CONSTRUCTOR(CLASS_NAME) \
+    FORCE_DEFAULT_CONSTRUCTORS(CLASS_NAME) \
+    \
+    CLASS_NAME(auto &&... arg) \
+        : raw(std::forward<decltype(arg)>(arg)...) \
+    { }