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_virtual_functions.cpp

@@ -148,6 +148,7 @@ class NCVirtTrampoline : public NCVirt {
 struct Base {
     /* for some reason MSVC2015 can't compile this if the function is pure virtual */
     virtual std::string dispatch() const { return {}; };
+    virtual ~Base() = default;
 };
 
 struct DispatchIssue : Base {
@@ -259,6 +260,7 @@ TEST_SUBMODULE(virtual_functions, m) {
         virtual std::string &str_ref() { return v; }
         virtual A A_value() { return a; }
         virtual A &A_ref() { return a; }
+        virtual ~OverrideTest() = default;
     };
 
     class PyOverrideTest : public OverrideTest {
@@ -311,6 +313,7 @@ public: \
         return say_something(1) + " " + std::to_string(unlucky_number()); \
     }
 A_METHODS
+    virtual ~A_Repeat() = default;
 };
 class B_Repeat : public A_Repeat {
 #define B_METHODS \
@@ -335,7 +338,7 @@ D_METHODS
 };
 
 // Base classes for templated inheritance trampolines.  Identical to the repeat-everything version:
-class A_Tpl { A_METHODS };
+class A_Tpl { A_METHODS; virtual ~A_Tpl() = default; };
 class B_Tpl : public A_Tpl { B_METHODS };
 class C_Tpl : public B_Tpl { C_METHODS };
 class D_Tpl : public C_Tpl { D_METHODS };