Allow type_casters to expose temporary dependencies

[jagerman]

The Eigen::Ref caster, in particular, requires that a temporary array be
kept alive in some cases. This allows a type caster to expose a
keep-alive dependency by providing a handle subcaster_keepalive()
method so that stl.h casters (which employ temporary subcasters during
loading) can hold onto any required extra dependencies.

Fixes #916

[dean0x7d]

Perhaps this might work as an alternative: make the temporary object a patient of the original handle src which is being loaded (the simpler weakref variant of the keep_alive mechanism should be enough here). That should ensure correct lifetime with the upside that supercasters would not need to be aware of the extra dependencies. (This should also work for the string_view caster and remove the extra object from the struct.)

Edit: Ah, perhaps not. That might be too much lifetime, letting the temporary live far longer than the caster.

[jagerman]

Closing in favour of #924.