Support multiple inheritance from python

This commit allows multiple inheritance of pybind11 classes from
Python, e.g.

    class MyType(Base1, Base2):
        def __init__(self):
            Base1.__init__(self)
            Base2.__init__(self)

where Base1 and Base2 are pybind11-exported classes.

This requires collapsing the various builtin base objects
(pybind11_object_56, ...) introduced in 2.1 into a single
pybind11_object of a fixed size; this fixed size object allocates enough
space to contain either a simple object (one base class & small* holder
instance), or a pointer to a new allocation that can contain an
arbitrary number of base classes and holders, with holder size
unrestricted.

* "small" here means having a sizeof() of at most 2 pointers, which is
enough to fit unique_ptr (sizeof is 1 ptr) and shared_ptr (sizeof is 2
ptrs).

To minimize the performance impact, this repurposes
`internals::registered_types_py` to store a vector of pybind-registered
base types.  For direct-use pybind types (e.g. the `PyA` for a C++ `A`)
this is simply storing the same thing as before, but now in a vector;
for Python-side inherited types, the map lets us avoid having to do a
base class traversal as long as we've seen the class before.  The
change to vector is needed for multiple inheritance: Python types
inheriting from multiple registered bases have one entry per base.

docs/advanced/classes.rst

@@ -619,27 +619,19 @@ interspersed with alias types and holder types (discussed earlier in this
 document)---pybind11 will automatically find out which is which. The only
 requirement is that the first template argument is the type to be declared.
 
-There are two caveats regarding the implementation of this feature:
-
-1. When only one base type is specified for a C++ type that actually has
-   multiple bases, pybind11 will assume that it does not participate in
-   multiple inheritance, which can lead to undefined behavior. In such cases,
-   add the tag ``multiple_inheritance``:
-
-    .. code-block:: cpp
-
-        py::class_<MyType, BaseType2>(m, "MyType", py::multiple_inheritance());
-
-   The tag is redundant and does not need to be specified when multiple base
-   types are listed.
-
-2. As was previously discussed in the section on :ref:`overriding_virtuals`, it
-   is easy to create Python types that derive from C++ classes. It is even
-   possible to make use of multiple inheritance to declare a Python class which
-   has e.g. a C++ and a Python class as bases. However, any attempt to create a
-   type that has *two or more* C++ classes in its hierarchy of base types will
-   fail with a fatal error message: ``TypeError: multiple bases have instance
-   lay-out conflict``. Core Python types that are implemented in C (e.g.
-   ``dict``, ``list``, ``Exception``, etc.) also fall under this combination
-   and cannot be combined with C++ types bound using pybind11 via multiple
-   inheritance.
+It is also permitted to inherit multiply from exported C++ classes in Python,
+as well as inheriting from multiple Python and/or pybind-exported classes.
+
+There is one caveat regarding the implementation of this feature:
+
+When only one base type is specified for a C++ type that actually has multiple
+bases, pybind11 will assume that it does not participate in multiple
+inheritance, which can lead to undefined behavior. In such cases, add the tag
+``multiple_inheritance`` to the class constructor:
+
+.. code-block:: cpp
+
+    py::class_<MyType, BaseType2>(m, "MyType", py::multiple_inheritance());
+
+The tag is redundant and does not need to be specified when multiple base types
+are listed.

include/pybind11/attr.h

@@ -210,17 +210,17 @@ struct type_record {
     /// How large is the underlying C++ type?
     size_t type_size = 0;
 
-    /// How large is pybind11::instance<type>?
-    size_t instance_size = 0;
+    /// How large is the type's holder?
+    size_t holder_size = 0;
 
     /// The global operator new can be overridden with a class-specific variant
     void *(*operator_new)(size_t) = ::operator new;
 
     /// Function pointer to class_<..>::init_holder
-    void (*init_holder)(PyObject *, const void *) = nullptr;
+    void (*init_holder)(instance *, const void *) = nullptr;
 
     /// Function pointer to class_<..>::dealloc
-    void (*dealloc)(PyObject *) = nullptr;
+    void (*dealloc)(const detail::value_and_holder &) = nullptr;
 
     /// List of base classes of the newly created type
     list bases;