Fix template trampoline overload lookup failure

Problem
=======

The template trampoline pattern documented in PR #322 has a problem with
virtual method overloads in intermediate classes in the inheritance
chain between the trampoline class and the base class.

For example, consider the following inheritance structure, where `B` is
the actual class, `PyB<B>` is the trampoline class, and `PyA<B>` is an
intermediate class adding A's methods into the trampoline:

    PyB<B> -> PyA<B> -> B -> A

Suppose PyA<B> has a method `some_method()` with a PYBIND11_OVERLOAD in
it to overload the virtual `A::some_method()`.  If a Python class `C` is
defined that inherits from the pybind11-registered `B` and tries to
provide an overriding `some_method()`, the PYBIND11_OVERLOADs declared
in PyA<B> fails to find this overloaded method, and thus never invoke it
(or, if pure virtual and not overridden in PyB<B>, raises an exception).

This happens because the base (internal) `PYBIND11_OVERLOAD_INT` macro
simply calls `get_overload(this, name)`; `get_overload()` then uses the
inferred type of `this` to do a type lookup in `registered_types_cpp`.
This is where it fails: `this` will be a `PyA<B> *`, but `PyA<B>` is
neither the base type (`B`) nor the trampoline type (`PyB<B>`).  As a
result, the overload fails and we get a failed overload lookup.

The fix
=======

The fix is relatively simple: we can cast `this` passed to
`get_overload()` to a `const B *`, which lets get_overload look up the
correct class.  Since trampoline classes should be derived from `B`
classes anyway, this cast should be perfectly safe.

This does require adding the class name as an argument to the
PYBIND11_OVERLOAD_INT macro, but leaves the public macro signatures
unchanged.

tests/test_virtual_functions.py

@@ -69,20 +69,24 @@ def test_inheriting_repeat():
     obj = VI_AR()
     assert obj.say_something(3) == "hihihi"
     assert obj.unlucky_number() == 99
+    assert obj.say_everything() == "hi 99"
 
     obj = VI_AT()
     assert obj.say_something(3) == "hihihi"
     assert obj.unlucky_number() == 999
+    assert obj.say_everything() == "hi 999"
 
     for obj in [B_Repeat(), B_Tpl()]:
         assert obj.say_something(3) == "B says hi 3 times"
         assert obj.unlucky_number() == 13
         assert obj.lucky_number() == 7.0
+        assert obj.say_everything() == "B says hi 1 times 13"
 
     for obj in [C_Repeat(), C_Tpl()]:
         assert obj.say_something(3) == "B says hi 3 times"
         assert obj.unlucky_number() == 4444
         assert obj.lucky_number() == 888.0
+        assert obj.say_everything() == "B says hi 1 times 4444"
 
     class VI_CR(C_Repeat):
         def lucky_number(self):
@@ -92,6 +96,7 @@ def test_inheriting_repeat():
     assert obj.say_something(3) == "B says hi 3 times"
     assert obj.unlucky_number() == 4444
     assert obj.lucky_number() == 889.25
+    assert obj.say_everything() == "B says hi 1 times 4444"
 
     class VI_CT(C_Tpl):
         pass
@@ -100,6 +105,7 @@ def test_inheriting_repeat():
     assert obj.say_something(3) == "B says hi 3 times"
     assert obj.unlucky_number() == 4444
     assert obj.lucky_number() == 888.0
+    assert obj.say_everything() == "B says hi 1 times 4444"
 
     class VI_CCR(VI_CR):
         def lucky_number(self):
@@ -109,6 +115,7 @@ def test_inheriting_repeat():
     assert obj.say_something(3) == "B says hi 3 times"
     assert obj.unlucky_number() == 4444
     assert obj.lucky_number() == 8892.5
+    assert obj.say_everything() == "B says hi 1 times 4444"
 
     class VI_CCT(VI_CT):
         def lucky_number(self):
@@ -118,6 +125,7 @@ def test_inheriting_repeat():
     assert obj.say_something(3) == "B says hi 3 times"
     assert obj.unlucky_number() == 4444
     assert obj.lucky_number() == 888000.0
+    assert obj.say_everything() == "B says hi 1 times 4444"
 
     class VI_DR(D_Repeat):
         def unlucky_number(self):
@@ -130,11 +138,13 @@ def test_inheriting_repeat():
         assert obj.say_something(3) == "B says hi 3 times"
         assert obj.unlucky_number() == 4444
         assert obj.lucky_number() == 888.0
+        assert obj.say_everything() == "B says hi 1 times 4444"
 
     obj = VI_DR()
     assert obj.say_something(3) == "B says hi 3 times"
     assert obj.unlucky_number() == 123
     assert obj.lucky_number() == 42.0
+    assert obj.say_everything() == "B says hi 1 times 123"
 
     class VI_DT(D_Tpl):
         def say_something(self, times):
@@ -150,6 +160,28 @@ def test_inheriting_repeat():
     assert obj.say_something(3) == "VI_DT says: quack quack quack"
     assert obj.unlucky_number() == 1234
     assert obj.lucky_number() == -4.25
+    assert obj.say_everything() == "VI_DT says: quack 1234"
+
+    class VI_DT2(VI_DT):
+        def say_something(self, times):
+            return "VI_DT2: " + ('QUACK' * times)
+
+        def unlucky_number(self):
+            return -3
+
+    class VI_BT(B_Tpl):
+        def say_something(self, times):
+            return "VI_BT" * times
+        def unlucky_number(self):
+            return -7
+        def lucky_number(self):
+            return -1.375
+
+    obj = VI_BT()
+    assert obj.say_something(3) == "VI_BTVI_BTVI_BT"
+    assert obj.unlucky_number() == -7
+    assert obj.lucky_number() == -1.375
+    assert obj.say_everything() == "VI_BT -7"
 
 @pytest.mark.skipif(not hasattr(pybind11_tests, 'NCVirt'),
                     reason="NCVirt test broken on ICPC")