Add static_assert to holder casters

[jagerman]

The holder casters assume but don't check that a holder<type>'s type is really a type_caster_base<type>; this adds a static_assert to make sure this is really the case, to turn things like std::shared_ptr<array> into a compilation failure.

Fixes #785

There is some merit to making the holder casters use a proper type_caster<type> rather than a type_caster_base<type> so that you could, for example, have a std::shared_ptr<std::string> or std::unique_ptr<Eigen::MatrixXd>. With this commit, holder arguments around non-generic type casters fails at compile time; before this commit, those failures show up in strange runtime failures (due to invoking the generic type caster rather than the specialized type caster).

[jagerman]

Just to be clear: I did not change this to use proper type casters because it would require a fairly substantial rewrite of the holder casters, which are currently quite closely tied to type_caster_base<type> internals.

[wjakob]

+1 That seems like a really nice way to catch this early and avoid confusion.

[jagerman]

Actually I don't think this really solves #785; this just changes the error to a compile-time failure. We're still unable to handle a holder around a type with a custom converter, which I think is going to happen in #785 when he switches to composition rather than inheritance.

[jagerman]

I'm going to merge this to take care of the current internal error issue; I opened #787 for the second part of this.