This adds a check when registering a class or a function with a holder
return that the same wrapped type hasn't been previously seen using a
different holder type.

This fixes pybind#1138 by detecting the failure; currently attempting to use
two different holder types (e.g. a unique_ptr<T> and shared_ptr<T>) in
difference places can segfault because we don't have any type safety on
the holder instances.

…type_info

…stered type_info

Having replaced the global map holders_seen, we can only check return values against already
registered types. Hence, we need to replace the check at initialization time with a check at call
time (when casting the return value).

A derived class needs to use the same holder type as its base class(es).
So far, the check was constrained to the default holder vs. custom holder types.
Thus, we replace the simple check (based on the default_holder flag) with
a more elaborate one, comparing holder type names.

…stered type_info

Improve wording and variable name:
- seen_holder_name -> holder_name
- seen holder -> declared holder

…in registered type_info

Runtime costs of checking holder compatibility for function return types during can be limited to
definition time (instead of calling time), if we require types to be registered before defining a function.
This allows to lookup the type_info record and validate the holder type as expected.
As we cannot call functions with an unregistered return type anyway, I think this is a good compromise.

…r_type in registered type_info

Simplify code using type_id<Return>()

To avoid costly runtime checks at function call time, compatibility of
holder types is now checked once at functionn registration time.
This assumes that all custom argument types are declared in advance!