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/pybind11_tests.h

@@ -1,6 +1,11 @@
 #pragma once
 #include <pybind11/pybind11.h>
 
+#if defined(_MSC_VER) && _MSC_VER < 1910
+// We get some really long type names here which causes MSVC 2015 to emit warnings
+#  pragma warning(disable: 4503) // warning C4503: decorated name length exceeded, name was truncated
+#endif
+
 namespace py = pybind11;
 using namespace pybind11::literals;
 
@@ -43,3 +48,17 @@ public:
     IncType &operator=(const IncType &) = delete;
     IncType &operator=(IncType &&) = delete;
 };
+
+/// Custom cast-only type that casts to a string "rvalue" or "lvalue" depending on the cast context.
+/// Used to test recursive casters (e.g. std::tuple, stl containers).
+struct RValueCaster {};
+NAMESPACE_BEGIN(pybind11)
+NAMESPACE_BEGIN(detail)
+template<> class type_caster<RValueCaster> {
+public:
+    PYBIND11_TYPE_CASTER(RValueCaster, _("RValueCaster"));
+    static handle cast(RValueCaster &&, return_value_policy, handle) { return py::str("rvalue").release(); }
+    static handle cast(const RValueCaster &, return_value_policy, handle) { return py::str("lvalue").release(); }
+};
+NAMESPACE_END(detail)
+NAMESPACE_END(pybind11)