Move /src to /subprojects

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

subprojects/libexpr/attr-set.cc

@@ -0,0 +1,53 @@
+#include "attr-set.hh"
+#include "eval-inline.hh"
+
+#include <algorithm>
+
+
+namespace nix {
+
+
+
+/* Allocate a new array of attributes for an attribute set with a specific
+   capacity. The space is implicitly reserved after the Bindings
+   structure. */
+Bindings * EvalState::allocBindings(size_t capacity)
+{
+    if (capacity == 0)
+        return &emptyBindings;
+    if (capacity > std::numeric_limits<Bindings::size_t>::max())
+        throw Error("attribute set of size %d is too big", capacity);
+    nrAttrsets++;
+    nrAttrsInAttrsets += capacity;
+    return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings((Bindings::size_t) capacity);
+}
+
+
+Value & BindingsBuilder::alloc(Symbol name, PosIdx pos)
+{
+    auto value = state.allocValue();
+    bindings->push_back(Attr(name, value, pos));
+    return *value;
+}
+
+
+Value & BindingsBuilder::alloc(std::string_view name, PosIdx pos)
+{
+    return alloc(state.symbols.create(name), pos);
+}
+
+
+void Bindings::sort()
+{
+    if (size_) std::sort(begin(), end());
+}
+
+
+Value & Value::mkAttrs(BindingsBuilder & bindings)
+{
+    mkAttrs(bindings.finish());
+    return *this;
+}
+
+
+}