When defining method from a member function pointer (e.g. `.def("f",
&Derived::f)`) we run into a problem if `&Derived::f` is actually
implemented in some base class `Base` when `Base` isn't
pybind-registered.

This happens because the class type is deduced from the member function
pointer, which then becomes a lambda with first argument this deduced
type.  For a base class implementation, the deduced type is `Base`, not
`Derived`, and so we generate and registered an overload which takes a
`Base *` as first argument.  Trying to call this fails if `Base` isn't
registered (e.g.  because it's an implementation detail class that isn't
intended to be exposed to Python) because the type caster for an
unregistered type always fails.

This commit adds a `method_adaptor` function that rebinds a member
function to a derived type member function and otherwise (i.e. regular
functions/lambda) leaves the argument as-is.  This is now used for class
definitions so that they are bound with type being registered rather
than a potential base type.

A closely related fix in this commit is to similarly update the lambdas
used for `def_readwrite` (and related) to bind to the class type being
registered rather than the deduced type so that registering a property
that resolves to a base class member similarly generates a usable
function.

Fixes #854, #910.

Co-Authored-By: Dean Moldovan <dean0x7d@gmail.com>