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_class.py

@@ -141,11 +141,8 @@ def test_operator_new_delete(capture):
         d = m.HasOpNewDelBoth()
     assert capture == """
         A new 8
-        A placement-new 8
         B new 4
-        B placement-new 4
         D new 32
-        D placement-new 32
     """
     sz_alias = str(m.AliasedHasOpNewDelSize.size_alias)
     sz_noalias = str(m.AliasedHasOpNewDelSize.size_noalias)
@@ -153,8 +150,8 @@ def test_operator_new_delete(capture):
         c = m.AliasedHasOpNewDelSize()
         c2 = SubAliased()
     assert capture == (
-        "C new " + sz_alias + "\nC placement-new " + sz_noalias + "\n" +
-        "C new " + sz_alias + "\nC placement-new " + sz_alias + "\n"
+        "C new " + sz_noalias + "\n" +
+        "C new " + sz_alias + "\n"
     )
 
     with capture: