Move /src to /subprojects

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

subprojects/libexpr/primops.hh

@@ -0,0 +1,56 @@
+#pragma once
+///@file
+
+#include "eval.hh"
+
+#include <tuple>
+#include <vector>
+
+namespace nix {
+
+/**
+ * For functions where we do not expect deep recursion, we can use a sizable
+ * part of the stack a free allocation space.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t nonRecursiveStackReservation = 128;
+
+/**
+ * Functions that maybe applied to self-similar inputs, such as concatMap on a
+ * tree, should reserve a smaller part of the stack for allocation.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t conservativeStackReservation = 16;
+
+struct RegisterPrimOp
+{
+    typedef std::vector<PrimOp> PrimOps;
+    static PrimOps * primOps;
+
+    /**
+     * You can register a constant by passing an arity of 0. fun
+     * will get called during EvalState initialization, so there
+     * may be primops not yet added and builtins is not yet sorted.
+     */
+    RegisterPrimOp(PrimOp && primOp);
+};
+
+/* These primops are disabled without enableNativeCode, but plugins
+   may wish to use them in limited contexts without globally enabling
+   them. */
+
+/**
+ * Load a ValueInitializer from a DSO and return whatever it initializes
+ */
+void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Value & v);
+
+/**
+ * Execute a program and parse its output
+ */
+void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v);
+
+void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column);
+
+}