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.

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>