stl.h: propagate return value policies to type-specific casters (#1455)

* stl.h: propagate return value policies to type-specific casters

Return value policies for containers like those handled in in 'stl.h'
are currently broken.

The problem is that detail::return_value_policy_override<C>::policy()
always returns 'move' when given a non-pointer/reference type, e.g.
'std::vector<...>'.

This is sensible behavior for custom types that are exposed via
'py::class_<>', but it does not make sense for types that are handled by
other type casters (STL containers, Eigen matrices, etc.).

This commit changes the behavior so that
detail::return_value_policy_override only becomes active when the type
caster derives from type_caster_generic.

Furthermore, the override logic is called recursively in STL type
casters to enable key/value-specific behavior.

tests/test_stl.py

@@ -2,6 +2,7 @@ import pytest
 
 from pybind11_tests import stl as m
 from pybind11_tests import UserType
+from pybind11_tests import ConstructorStats
 
 
 def test_vector(doc):
@@ -198,3 +199,12 @@ def test_missing_header_message():
     with pytest.raises(TypeError) as excinfo:
         cm.missing_header_return()
     assert expected_message in str(excinfo.value)
+
+
+def test_stl_ownership():
+    cstats = ConstructorStats.get(m.Placeholder)
+    assert cstats.alive() == 0
+    r = m.test_stl_ownership()
+    assert len(r) == 1
+    del r
+    assert cstats.alive() == 0