Use rvalue subcasting when casting an rvalue container

This updates the std::tuple, std::pair and `stl.h` type casters to
forward their contained value according to whether the container being
cast is an lvalue or rvalue reference.  This fixes an issue where
subcaster casts were always called with a const lvalue which meant
nested type casters didn't have the desired `cast()` overload invoked.
For example, this caused Eigen values in a tuple to end up with a
readonly flag (issue #935) and made it impossible to return a container
of move-only types (issue #853).

This fixes both issues by adding templated universal reference `cast()`
methods to the various container types that forward container elements
according to the container reference type.

tests/test_stl.cpp

@@ -61,6 +61,46 @@ TEST_SUBMODULE(stl, m) {
         return set.count("key1") && set.count("key2") && set.count("key3");
     });
 
+    // test_recursive_casting
+    m.def("cast_rv_vector", []() { return std::vector<RValueCaster>{2}; });
+    m.def("cast_rv_array", []() { return std::array<RValueCaster, 3>(); });
+    // NB: map and set keys are `const`, so while we technically do move them (as `const Type &&`),
+    // casters don't typically do anything with that, which means they fall to the `const Type &`
+    // caster.
+    m.def("cast_rv_map", []() { return std::unordered_map<std::string, RValueCaster>{{"a", RValueCaster{}}}; });
+    m.def("cast_rv_nested", []() {
+        std::vector<std::array<std::list<std::unordered_map<std::string, RValueCaster>>, 2>> v;
+        v.emplace_back(); // add an array
+        v.back()[0].emplace_back(); // add a map to the array
+        v.back()[0].back().emplace("b", RValueCaster{});
+        v.back()[0].back().emplace("c", RValueCaster{});
+        v.back()[1].emplace_back(); // add a map to the array
+        v.back()[1].back().emplace("a", RValueCaster{});
+        return v;
+    });
+    static std::vector<RValueCaster> lvv{2};
+    static std::array<RValueCaster, 2> lva;
+    static std::unordered_map<std::string, RValueCaster> lvm{{"a", RValueCaster{}}, {"b", RValueCaster{}}};
+    static std::unordered_map<std::string, std::vector<std::list<std::array<RValueCaster, 2>>>> lvn;
+    lvn["a"].emplace_back(); // add a list
+    lvn["a"].back().emplace_back(); // add an array
+    lvn["a"].emplace_back(); // another list
+    lvn["a"].back().emplace_back(); // add an array
+    lvn["b"].emplace_back(); // add a list
+    lvn["b"].back().emplace_back(); // add an array
+    lvn["b"].back().emplace_back(); // add another array
+    m.def("cast_lv_vector", []() -> const decltype(lvv) & { return lvv; });
+    m.def("cast_lv_array", []() -> const decltype(lva) & { return lva; });
+    m.def("cast_lv_map", []() -> const decltype(lvm) & { return lvm; });
+    m.def("cast_lv_nested", []() -> const decltype(lvn) & { return lvn; });
+    // #853:
+    m.def("cast_unique_ptr_vector", []() {
+        std::vector<std::unique_ptr<UserType>> v;
+        v.emplace_back(new UserType{7});
+        v.emplace_back(new UserType{42});
+        return v;
+    });
+
     struct MoveOutContainer {
         struct Value { int value; };