Implement py::init_alias<>() constructors

This commit adds support for forcing alias type initialization by
defining constructors with `py::init_alias<arg1, arg2>()` instead of
`py::init<arg1, arg2>()`.  Currently py::init<> only results in Alias
initialization if the type is extended in python, or the given
arguments can't be used to construct the base type, but can be used to
construct the alias.  py::init_alias<>, in contrast, always invokes the
constructor of the alias type.

It looks like this was already the intention of
`py::detail::init_alias`, which was forward-declared in
86d825f330, but was apparently never
finished: despite the existance of a .def method accepting it, the
`detail::init_alias` class isn't actually defined anywhere.

This commit completes the feature (or possibly repurposes it), allowing
declaration of classes that will always initialize the trampoline which
is (as I argued in #397) sometimes useful.

tests/test_issues.cpp

@@ -117,30 +117,6 @@ void init_issues(py::module &m) {
     m2.def("expect_float", [](float f) { return f; });
     m2.def("expect_int", [](int i) { return i; });
 
-    // (no id): don't invoke Python dispatch code when instantiating C++
-    // classes that were not extended on the Python side
-    struct A {
-        virtual ~A() {}
-        virtual void f() { py::print("A.f()"); }
-    };
-
-    struct PyA : A {
-        PyA() { py::print("PyA.PyA()"); }
-
-        void f() override {
-            py::print("PyA.f()");
-            PYBIND11_OVERLOAD(void, A, f);
-        }
-    };
-
-    auto call_f = [](A *a) { a->f(); };
-
-    pybind11::class_<A, std::unique_ptr<A>, PyA>(m2, "A")
-        .def(py::init<>())
-        .def("f", &A::f);
-
-    m2.def("call_f", call_f);
-
     try {
         py::class_<Placeholder>(m2, "Placeholder");
         throw std::logic_error("Expected an exception!");