Move /src to /subprojects

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

subprojects/libexpr/gc-small-vector.hh

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <boost/container/small_vector.hpp>
+
+#include "value.hh"
+
+namespace nix {
+
+/**
+ * A GC compatible vector that may used a reserved portion of `nItems` on the stack instead of allocating on the heap.
+ */
+template <typename T, size_t nItems>
+using SmallVector = boost::container::small_vector<T, nItems, traceable_allocator<T>>;
+
+/**
+ * A vector of value pointers. See `SmallVector`.
+ */
+template <size_t nItems>
+using SmallValueVector = SmallVector<Value *, nItems>;
+
+/**
+ * A vector of values that must not be referenced after the vector is destroyed.
+ *
+ * See also `SmallValueVector`.
+ */
+template <size_t nItems>
+using SmallTemporaryValueVector = SmallVector<Value, nItems>;
+
+}