Reimplement py::init<...> to use common factory code

This reimplements the py::init<...> implementations using the various
functions added to support `py::init(...)`, and moves the implementing
structs into `detail/init.h` from `pybind11.h`.  It doesn't simply use a
factory directly, as this is a very common case and implementation
without an extra lambda call is a small but useful optimization.

This, combined with the previous lazy initialization, also avoids
needing placement new for `py::init<...>()` construction: such
construction now occurs via an ordinary `new Type(...)`.

A consequence of this is that it also fixes a potential bug when using
multiple inheritance from Python: it was very easy to write classes
that double-initialize an existing instance which had the potential to
leak for non-pod classes.  With the new implementation, an attempt to
call `__init__` on an already-initialized object is now ignored.  (This
was already done in the previous commit for factory constructors).

This change exposed a few warnings (fixed here) from deleting a pointer
to a base class with virtual functions but without a virtual destructor.
These look like legitimate warnings that we shouldn't suppress; this
adds virtual destructors to the appropriate classes.

tests/test_factory_constructors.py

@@ -284,7 +284,19 @@ def test_init_factory_dual():
 
 
 def test_no_placement_new(capture):
-    """Tests a workaround for `py::init<...>` with a class that doesn't support placement new."""
+    """Prior to 2.2, `py::init<...>` relied on the type supporting placement
+    new; this tests a class without placement new support."""
+    with capture:
+        a = m.NoPlacementNew(123)
+
+    found = re.search(r'^operator new called, returning (\d+)\n$', str(capture))
+    assert found
+    assert a.i == 123
+    with capture:
+        del a
+        pytest.gc_collect()
+    assert capture == "operator delete called on " + found.group(1)
+
     with capture:
         b = m.NoPlacementNew()