slightly tweak implementation of empty collection nodes

This change removes the `const` qualifier from the static objects
containing empty rrbtree and champ nodes, and also modifies the methods
slightly so that they return a reference. This change makes it possible
for these types to be used as part of a garbage-collected runtime which
makes use of a copying collector, because if the user specifies a
heap_poicy which allocates these nodes into a heap that needs to be
relocated during garbage collection, the garbage collector will need to
update the static pointer inside these methods in order to correctly
relocate these nodes.

This change removes the `const` qualifier from the static
objects containing empty rrbtrees and champ

immer/detail/hamts/champ.hpp

@@ -34,10 +34,11 @@ struct champ
     node_t* root;
     size_t size;
 
-    static node_t* empty()
+    static node_t*& empty()
     {
-        static const auto node = node_t::make_inner_n(0);
-        return node->inc();
+        static auto node = node_t::make_inner_n(0);
+        node->inc();
+        return node;
     }
 
     champ(node_t* r, size_t sz = 0)

immer/detail/rbts/rrbtree.hpp

@@ -47,16 +47,18 @@ struct rrbtree
                ipow((size_t{1} << B) - 2, (S - BL) / B);
     }
 
-    static node_t* empty_root()
+    static node_t*& empty_root()
     {
-        static const auto empty_ = node_t::make_inner_n(0u);
-        return empty_->inc();
+        static auto empty_ = node_t::make_inner_n(0u);
+        empty_->inc();
+        return empty_;
     }
 
-    static node_t* empty_tail()
+    static node_t*& empty_tail()
     {
-        static const auto empty_ = node_t::make_leaf_n(0u);
-        return empty_->inc();
+        static auto empty_ = node_t::make_leaf_n(0u);
+        empty_->inc();
+        return empty_;
     }
 
     template <typename U>