def_readwrite/readonly: bind using class type, not inferred type

[jagerman]

Current def_readwrite/def_readonly create getter/setter lambdas that
take the inferred type C as the first argument, but this is a problem
if the member is actually a base class member (e.g.

derived.def_readonly("x", &Base::x)

because this ends up creating a setter/getter that aren't callable with
a derived instance when Base isn't registered: it invariably produces
an invalid argument failure when attempting to access the property due
to the D instance/B argument mismatch.

This commit changes the generated lambdas to use the type (as given in
class_<type>) rather than the inferred C for the lambdas, and adds a
static_assert failure for an attempt to call it with a member that isn't
part of type (or a base class).

Fixes #910

[bmerry]

Isn't this already fixed by #855?

[jagerman]

Yes, if #855 is merged it fixes this as well. There is some concern than #855 is too complicated, however, so if we decide to drop it, this is a partial alternative.

[jagerman]

I've simplified #855; closing this in favour of it.